It has a limit parameter so you can limit the speed. Great if you don't want to saturate some link or have additional costs for uploading above a certain rate per hour/day.

Also useful for testing behaviour on slow filesystem / connections.

It can take a pid argument too, -d IIRC, which will get it to display progress info for all the open file descriptors of a running process.

Really useful as a quick way to check what a IO process is doing if appears to be stuck.