I tape my mouth because I hated waking up with a dry mouth that tasted horrible. I've also read that this contributes to tooth decay.

It's just a small piece of tape, as big as half a fingernail. It's placed on my lips right in the middle, and it works more by reminding my brain to keep my mouth closed and less by actually mechanically holding my mouth shut. If my nose is stuffed, I can just open a slight gap between my lips to breathe through. Otherwise, though, when my mouth is that close I will automatically close my lips and breathe through my nose through the night, which anecdotally improves my sleep quality and reduces morning breath.

My girlfriend had jaw issues and was recommended this after a surgery by her doctor. I decided to give it a try aswell as I had heard about the potential benefits. Used to wake up with a terrible dryness and taste in my mouth. Doesnt happen anymore now. Strangely enough my dreams got more vivid. Also, eventually it became like such a ritual that I would feel like something was missing if I didn't tape up. It started to feel snug, like a weighted blanket.

I met a guy who did this to prevent snoring.

Used to have a chronically stuffy nose at night, maybe due to allergies. Probably thats when the bad habit started.

I guess a chin strap would work aswell.