[alwyn]

In my view one reason is to ensure integrity down the line. You want the checksum of a file to still be the same when you download it maybe years later. If it isn't, you get warned about it. Without the checksum, how will you know for sure? Keep your own database of checksums? :)

[supriyo-biswas]

If we're talking about bitrot protection, I'm pretty sure S3 would use some form of checksum (such as crc32 or xxhash) on each internal block to facilitate the Reed-Solomon process.

If it's verifying whether if it's the same file, you can use the Etag header which is computed server side by S3. Although I don't like this design as it ossifies the checksum algorithm.

[everfrustrated]

You may be interested in this https://aws.amazon.com/blogs/aws/introducing-default-data-in...