1. Cost. It might vary depending on vendor, but generally S3 is much cheaper than block storage, at the same time with some welcome guarantees (like 3 copies).

2. Pay for what you use.

3. Very easy to hand off URL to client rather than creating some kind of file server. Also works with uploads AFAIR.

4. Offloads traffic. Big files often are the main source of traffic on many websites. Using S3 allows to remove that burden. And S3 usually served by multiple servers which further increases speed.

5. Provider-independent. I think that every mature cloud offers S3 API.

I think that there are more reasons. Encryption, multi-region and so on. I didn't use those features. Of course you can implement everything with your own software, but reusing good implementation is a good idea for most projects. You don't rewrite postgres, so you don't rewrite S3.