It's programmable, so you're not just running useless hashes. And with Proof of Stake it's way more efficient than Bitcoin.
B)Specifically regarding self custody? Quite a few. Argent is one notable example https://argentwallet.io/
I'm simply curious as I've always seen defi as weird because nothing i've seen seemed useful.
Rocketpool is a project that allows users to contribute to the proof of stake system with very small amounts of capital, and share in the returns thereof, again trustlessly.
There are miles and miles of useful projects out there. Any truly permissionless ecosystem is going to be full of scams, that's the nature of the beast. It doesn't mean that there aren't extremely clever and elegant projects being built by very talented teams of developers trying to make something that reduces the amount of global suck.