Many uses with the 2 tools. I had in mind from one of your tutes where you just wanted to "remove" 1 surface so you could do a blend.

So with the trim you could hit your sphere:



Then select "Only" the surface closest to your blend to be removed, to leave the opposite side untouched:



Select the edges for the blend:



And Poof! A good reason to leave open holes on the surface:



Burr