the most simple answer is that you selected s3 as your sole storage backend for whatever services/apps you have deployed. But now, you want to integrate something that does not support s3 into your stack. So you'll look at stuff like this.

Now, why would you self host S3 in the first place:

The big reason is security. NFS is a security nightmare that believes connections when they claim to be root. It has no encryption and basically no authentication by default [1]

NBD is a little better, but both NBD and NFS have the problem in that services run in the kernel, exposing frightening attack surface to privileged parts of linux systems. Also, NBD is block storage and a lot less supported as a storage backend for various services.

S3 on the other hand, is actually designed to be exposed over the internet. http based, you can use https for encryption, and http basic auth or other methods for authentication trivially. So if you ever end up running something distributed over the internet, NFS is basically not an option, and S3 is a much better solution.

[1] Was posted to lemmy: https://programming.dev/post/34520407