I have Bauer speed plates in my hockey skates. They are heat mouldable arch supports and make a huge difference to my skating. I recently took them out, and noticed no difference at first, but after a while I felt less stable, so I put them back in and all is well again. I suspect that without the arch support, my feet sometimes slipped forwards, due to the arch collapsing, which gradually deformed the skates, thus ruining the fit. The support helps keep the foot in position and helps the skate mould to the foot.
Incidentally I don't recommend Superfeet though some people do like them. The yellow and I think the carbon are supposed to be suitable for skates.
I assume by arch supports the others mean some simple little inserts that literally go beneath the arches, presumably with some sticky stuff to hold them in place.
I don't have flat feet either.