The past did not use crc32 , it used content-md5 but not everywhere. It also did not support full object checksums for multipart. And here is the thing that was problematic: if you uploaded parts you could not check the object after it was uploaded since aws did not calculate a hash and safe it so that you can do a head call and compare your locally generated checksum with the one online. There are cases where generating a checksum up front is infeasible and using content-md5 it was not so easy / fast to chunk the upload and generating the crc while uploading. And here is the biggest benefit: The crc algorithm do not need to be concatinated in order. So basically you can parallelize the hash generation since you can hash 4kb chunks and concat them. And in some cases the sdk did not generate a checksum at all.

Edit: I forgot, since full object checksums are now the default, aws can now upload multiple parts in parallel, which was not possible before. (For upload multipart)