Why Devotees offer Ganges Water back to Ganges?

After taking one's bath in the Ganges, a devotee takes a palmful of water and offers it back to the Ganges. Actually, when one takes a palmful of water from the Ganges, the Ganges does not lose anything, and similarly if a devotee offers a palmful of water to the Ganges, the Ganges does not increase in any way. But by such an offering, the devotee becomes celebrated as a devotee of mother Ganges.

Similarly, when we offer anything with devotion and faith, what we offer does not belong to us, nor does it enrich the opulence of the Supreme Personality of Godhead.

But if one offers whatever he has in his possession, he becomes a recognized devotee.
In this regard, the example is given that when one's face is decorated with a garland and sandalwood pulp, the reflection of one's face in a mirror automatically becomes beautiful. The original source of everything is the Supreme Personality of Godhead, who is our original source also. Therefore when the Supreme Personality of Godhead is decorated, the devotees and all living entities are decorated automatically.

Actually, no one can give anything to the Supreme Personality of Godhead, for He is full in everything. For example, Bali Maharaja wanted to give charity to Lord Vamanadeva, but the Lord expanded His body in such a way that He showed Bali Maharaja that everything in the universe is already in His body.

Reference: Srimad Bhagavatam, Canto 8, Chapter 20, Purport of Text 21