Why should we worship Lord when everything is nothing but the Lord Himself?
The impersonalists argue that there is no use in worshiping the Lord when everything is nothing but the Lord Himself. The personalist, however, worships the Lord out of a great sense of gratitude, utilizing the ingredients born out of the bodily limbs of the Lord. The fruits and flowers are available from the body of the earth, and yet mother earth is worshiped by the sensible devotee with ingredients born from the earth. Similarly, mother Ganges is worshiped by the water of the Ganges, and yet the worshiper enjoys the result of such worship. Worship of the Lord is also performed by the ingredients born from the bodily limbs of the Lord, and yet the worshiper, who is himself a part of the Lord, achieves the result of devotional service to the Lord. While the impersonalist wrongly concludes that he is the Lord himself, the personalist, out of great gratitude, worships the Lord in devotional service, knowing perfectly well that nothing is different from the Lord. The devotee, therefore, endeavors to apply everything in the service of the Lord because he knows that everything is the property of the Lord and that no one can claim anything as one’s own. This perfect conception of oneness helps the worshiper in being engaged in His loving service, whereas the impersonalist, being falsely puffed up, remains a non-devotee forever, without being recognized by the Lord.
Source: A.C. Bhaktivedanta Swami Prabhupada (2014 edition), “Srimad Bhagavatam”, Second Canto, Chapter 6 – Text 23