Here's how I'm going to use it:

I run a service that process S3 and Cloudfront logs for people. Each S3 Bucket generates between 200 and 1000 logfiles every day that need to be combined together to for a full day's weblog.

Part of what my service does is re-upload that combined logfile back to the bucket in question, and since for large sites it can be upwards of 200mb zipped, it'd be nice to be able to upload it in little 5mb chunks that can be resent if anything goes wrong.