DeFi has concrete use cases. Just look at how derivatives and options work in DeFi. You will see how financial products work in traditional finance. You can understand them by reading their smart contract code. Another example would be reading the code of uniswap [0]. It is a decentralized exchange in other words, it lets you exchange your value without intermediaries and a central authority by having an algorithmic pricing. (Actually, by saying algorithmic, It is actually pretty simple pricing curve :) but it works :).
I hope it is clear.
thank you.