They're not but I'd definitely recommend one unless you're only running a couple of cables - even then, mounting a patch panel is easier IMO than mounting a faceplate. This is all assuming the equipment is out of sight though; if you're talking about an area where you don't want network equipment visible then a faceplate makes more sense.
A patch panel basically creates a partition between your in-wall infrastructure (solid core cabling, faceplates and patch panel) and your malleable equipment (patch cabling, switch, router, devices). You could bypass the patch panel and instead crimp RJ45 connectors straight onto your solid core cables, but solid core cabling isn't very malleable and isn't designed to be moved much, so you don't really want to be unplugging and plugging them in a lot. Another problem is your in-wall cabling has a fixed length. If you wanted to move your switch you're buggered. It's basically the same as asking "Why use faceplates? Why not just plug the solid core straight into my devices?"...not a great idea. Patch panels just make sense from a maintenance point of view.
Basic patch panels are cheap, I figure why give yourself a potential headache in the future when you can put up that partition and (hopefully) not have to touch anything beyond the patch panel again?