The same reason why when you use Venmo you don't think "wait, why use USD at all if it's all a database?"
The truth is, the current financial system is just a large number of databases trying to sync with one another, and one of cryptocurrencies' goal is to decrease this amount of databases.
It’s still backed by the currency. Kind of like how when you make a stock trade the actual trade isn’t cleared for some time. In this way the blockchain of Bitcoin is simply transparent and easy to follow, if slow and expensive.
If you never actually use the blockchain, then you don't really know if those BTC you 'bought' even exist.