Like KM said, if you have it set to automatically purge download sources after a time - which is the default - nothing will happen when you press it.
If you however have cleared that checkbox you will see lots of inactive sources in your downloads over time, which can then be cleared out by that button.
I never set it to automatically purge sources because some of them for tv stuff are hard to find and it's nice to see the name of the person who has that file and maybe even has a room tag which tells me where to find them.
I used to go hunting through all the tv rooms for some people from that list of sources, lol