It's probably the most important storage API in the industry. Implementing it gives you on-prem storage, AWS S3 (the Hoover Dam of Internet storage megaprojects, arguably the most reliable store of any kind available to any normal programmer), and a whole ecosystem of S3-compatible options with different features and price points.
It's a little like asking why you'd use SQL.
The S3 standard is certainly really important. It's perhaps the most important web standard without any sort of standards organization or formal spec (seriously, Amazon, I'm begging you to open up to ISO or IEC or SNIA or somebody).
And SQL is also very important. And yet, if somebody said "I need to store data, but it's not relational, and I just need 1000 rows, what's the best SQL solution," I would still ask why exactly they needed SQL. The might have a good reason (for example, SQLite can be a weirdly good way to persist application data), but I don't know it yet. That's why I asked.