If your copying or moving large amount of data from one hard disk to another disk / partition the reaction time of the affected disk can get so large, that you are unable to work with other data, which is on the same disk.
In many cases if you just limit the coping / moving speed of the file transfer working with other files on the same drive is still possible. Otherwise you have to decides if you either want to transfer the files or want to work with the other data.
If you can dynamically change the speed limit you can remove the limit if you are no longer working with the affected disk or set even higher restrictions if your other task requires higher IO load.