There are many reasons to do client-side encryption, some of them are that you want to storage the data on multiple storage providers but with the same key. A

There are many reasons to do client-side encryption, some of them are that you want to storage the data on multiple storage providers but with the same key. A national law of the country explicitly tells the company to do so, or a company you are in contract with asks of you to do so. The key that S3 can provide is not good enough for your internal usage,... Stop looking at everything pure technically because that is not how the real world works.