From my perspective, requesting high speed isn't because I am in a hurry necessarily, it's because due to air traffic control or departure traffic, I am higher than I would like to be, that said, most modern airliners have big wings, so consider an airplane descending at say 300 KTS (Power off), now approaching 10 K if I reduce speed 50 KTS my airplane turns into a glider, other options would be to use speed brakes which no one in the cabin appreciates, soooo by getting permission to "keep high speed" it allows me to continue the descent rate.
