Plex probably isn’t the best example, but yes, you can use Tailscale to create a sort of mesh network to access devices within private networks. Essentially any device that’s connected to tailscale can be contacted by other clients connected to tailscale. There are extra routing things you can do to use a tailscale device as a sort of “exit node”, but that’s the basic gist.

I don’t really use it for this, but here are some things I do use it for:

• metrics scraping on servers without needing to open ports or worry about ssl encryption. Works great for federating Prometheus instances or scraping exporters
• secure access to machines not directly exposed to the internet. I.e. ssh access to my home box while I’m traveling
• being an exit node for web traffic while traveling. I.e. maybe you are traveling and have a bank who is giving you grief about logging in – masquerade that connection from your home IP

I mostly just use it for metrics scraping though

Not Plex, but yes. I use it with Microsoft Remote Desktop if I need to access a work-related computer that I keep at home while traveling.

I also use it for the more typical use case of a cloud server that I can ssh into even though it exposes zero ports publicly.

Yes, exactly that.
I use it to SSH into my devices all the time.