There is an interesting story which explains why devotees offer such large amounts of valuable to Tirupati Balaji.
Goddess Lakshmi, also referred as Shri, once had a fight with Lord Vishnu and left Vaikunta. She came and settled on earth in disguise.
Lord Vishnu soon arrived on earth searching for Goddess Lakshmi.
But He failed to trace Shri and settled on Tirumala hills in form of a forest gatherer and continued his search. During the search, Lord Vishnu met a beautiful girl named Padmavati who was the daughter of the King of the seven hills in Tirumala. They both fell in love and decided to get married.
The father of Padmavati asked for a huge bridal price and to pay the money Lord Vishnu took a huge loan from Kubera, the Hindu god who is the treasurer of wealth. But Kubera gave the loan on the condition that Vishnu cannot return to Vaikunta (heavenly abode) without paying off the debt.
So Lord Vishnu resides at Tirumala as Tirupati Venkateswara without returning to Vaikunta. To help him repay his debt, devotees offer him wealth and in return Lord Vishnu fulfills their prayers.