For emulating vintage gaming hardware that's no longer available, one argument would be preservation of the media. But what's the rationale (not rationalization) on running an emulation of the Nintendo Switch, when it's still commercially available? In practice, is it usually because people want to play the games without paying for the hardware and software?
It seems far more likely that this is done just for the curiosity of doing it.
There’s one fascinating outcome to me as a result of Nintendo essentially repeating two hardware generations (GameCube -> Wii, then Wii U -> Switch), which is that on the highest end gaming PCs, games for the Switch can and do run at much higher resolutions and framerates than the Switch itself is capable of. On the YouTube videos I’ve seen the games don’t appear as stable as native hardware, but they really do look tremendous. Especially Breath of the Wild.