The English equivalents of 'Puja' are homage, reverence, worship, respect, etc. The system of Puja relates to religions, deities and God. In some religions there is an elaborate system of rituals for Puja while in some religions there is a simple method of prayer.
All the major religions of the world originated in Asia. South Asia is the birth place of four major religions- Hinduism, Buddhism, Jainism and Sikhism. All these religions originated in India. Christianity and Islam originated in South- west Asia for which Jerusalem, Mecca and Madina are important places. In all these religions, the concept of God exists. Only Sikhism concentrates on Gurus. But, the system of Puja and prayers is associated with all the religions.
This system is visible in today's world. Almost all the religions except Hinduism evolved during the last 2,600 years. Hinduism is the oldest religion of the world and its history is about 4,000 years long but Hinduism also developed after the 1st agricultural revolution which took place about 10,000 years ago and the intervening period was Vedic period in India during which such Puja systems had not taken roots.
In order to understand the purpose of Puja we will have to go back in the early stages of human civilizations when man was a jungle dweller and without shelter, because, the system of reverence to some unknown power started emerging in that period. This will define the causes also which drive human beings to perform Puja.
a) The Puja system emerged to get protection of unknown powers, from natural calamities, hazards and disasters in the early period:
In the initial stages man was a food gatherer, hunter, fisher and shelter-less living in jungle in small groups. There was no concept of God in that period. The small groups of people lived in isolation. However, they saw the impact of natural calamity on their lives. They experienced the impact of wild fires, thunderbolts, storms, heavy rains, floods, earthquakes, volcanic eruptions and they began fearing such phenomenon.
In order to protect themselves from such events, they must have visualized the role of an unknown superpower and started showing reverence to such powers. This culminated in the Vedic period in ancient India when elaborate system of worship of Fire God (Agni Dev), Wind God (Vayu Dev), Water God (Varun Dev), Sun God (Surya Dev), Thunderbolt God (Indra dev) began.
In addition to such deities a system of reverence was also invented to get protection from every element of the environment of the earth. Trees, soil, rivers, hills, animals, reptiles, birds, all became objects of reverence in the Vedic period. We may call it "Animism" which was the system of Puja up to the Vedic period.
b) The rise of Sakam Puja (worship with expectation of reward):
Since Hinduism developed from Vedic religion, there is a multiplicity of God's and Goddesses in the Hindu religion. There are 100's of Puja systems associated with deities. There is a multiplicity of religious books. The concept of "Tridev" (Trinity namely Bramha, Vishnu and Mahesh) developed in the early stages of Hinduism.
The system of worship was designed to please a particular deity for obtaining some blessings and rewards. This Puja is for fulfillment of human desires, wants, freedom from sufferings, for off-springs, for marriage, rise in status and position, for health, wealth, and other desires.
The ancient kings organized "Aswa-medha Yagya" for enlarging their kingdoms and empires. According to Ramayana even Raja Ram took recourse to this yagya and the horse of the yagya was caught by his children Lav and Kush which triggered a war between father and sons. Yagya is an elaborate system of Puja which is practised even today with a particular purpose.
However, this type of Puja is medium quality Puja, because it is done for material rewards. Lord Krishna in Shrimad Bhagwat Gita said as follows:-
"Antavat tu phalam tesam tad bhavaty alpa medhasam
devan deva yajo yanti mad bhakta yanti mam api"
(Chapter VII, Verse 23)
Translation: Men of small intelligence worship the demigods, and their fruits are limited and temporary. Those who worship the demigods go to the planets of the demigods, but my devotees ultimately reach my supreme planet.
c) System of Nishkam Puja (worship without attachment to expectations)
The Hinduism continued to transform through the ages and became more scientific. It came to be regarded as a religion associated with fundamental doctrine of Karma. This thought emerged during the battle of Mahabharata when Lord Krishna began counseling Arjun. The same preaching is found in Shrimad Bhagwat Gita.
According to Hindu doctrine, all beings have souls and there is a process of reincarnation after death. These souls are arranged in a hierarchy. The fundamental point is to move upward in the hierarchy and then escape from the eternal cycle of birth and death through union with the Brahma. The principle of reincarnation of soul is the cornerstone of Hinduism and every soul should try to attain salvation (Nirvana, Mukti, Moksha) through spiritual practises. This should be the supreme purpose behind Puja.
For this Lord Krishna showed the paths of "Karma Yoga" (adherence to duty while remaining detached from reward), "Gyan Yoga" (knowledge of the absolute) and "Bhakti Yoga" (spiritual practise of fostering loving devotion to God). Lord Krishna said in Bhagwat Gita:-
"a-brahma bhuvanal lokah punar avartino rjuna
mam upetya tu kaunteya punar janma na vidyate"
(Chapter VIII, Verse 16)
Translation: From the highest planet in the material world down to the lowest, all are places of misery wherein repeated birth and death take place. But one, who attains to my abode, O son of Kunti, never takes birth again.
d) Rise of Sadguru Puja (worship of ascendant masters):
Sadgurus (ascendant masters) are incarnations of God himself who live amidst us to guide us in our endeavor to reach to God. Lord Krishna in Bhagwat said that "Saints are incarnations of God". Sadgurus are known by different names in different religions. Hindus call them "Paramhansa" Muslims call them "Qutub" and Christians call them "the ascendant masters". They have incarnated in almost all religions. Some prominent names are "Jesus Christ", "Moses", "Gautum Buddha", "Lord Mahavir", "Aadi Sankracharya", and "Prophet Mohammad". The most recent incarnations of Sadgurus go by the name of "Ram Krishna Paramhansa", Shri Sai Nath of Shirdi", "Guru Nanak" etc.
The Sadguru guides his disciple to attain God for which Guru Bhakti (devotion to Guru) is the most important requirement. Therefore, the disciples are duty bound to keep their Guru in high reverence.
The life of the modern man has become very complex. He cannot follow the elaborate Puja systems and rituals followed by his ancestors. There is shortage of time and in these difficult moments; the only recourse left to human beings is to keep faith in Sadguru who guides them to spiritual betterment. Hence, worship of Sadguru is the most important Puja in today's world.