The term "web 3" can be a bit misleading. It sounds like the next version of the web. Like it will completely replace web 2. In reality, web 2 didn't even replace web 1. A lot of web 1 websites still exist.
Blockchain is the foundation of web 3. It's mainly just good for distributing rewards (money).
On web 1, you own your own server. On web 2, companies own servers and figure out how to make money to cover that cost. On web 3, anyone can contribute servers (or less powerful computers) to a network and get rewarded for doing so.
If you're building a "blockchain app," it's probably going to run on a combination of web 2 (or web 1) and web 3 infrastructure. You shouldn't put everything on the blockchain. The trick for developers is to figure out the right place to draw that line.
P.S. Yesterday, I called Koin a "coupon," but "membership" is a better analogy. Holding Koin is like having a blockchain membership that will never expire.