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
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