This is going to give the same reason as Ron Beyer's answer. I'm only writing another because I found his a little hard to follow and I'm going to try to explain it differently. It's nothing to do with mag fouling at all.
The magnetos are wired up so that a failure of the switch or wiring leaves them live (on), unlike most electrical devices which are wired to be off if they fail. This is normally what you want because if the wiring fails in-flight, you'd like the magnetos to keep working. But it can cause a dangerous state, where the magnetos are stuck on when you think they're off. If you swung the prop in that condition, it could fire when you're not expecting it.
So one at a time, we switch a magneto off, and check that the RPM appears to drop, and that the engine sound changes. The sound is more important - as John K says, the drop is small at the kind of RPM you do your mag drop test, and hard to see unambiguously on the gauge. This way, you know that switching it off really does isolate the magneto, so you know that when you turn both off after stopping the engine, the propeller is safe.
It's not so much for your benefit as whoever will touch or fly the aircraft next. Even so, you should always treat a propeller as live. If you have to swing it (e.g. to inspect it), do so as if you were hand-cranking the engine, keeping well clear and making sure your arm comes out of its path.