With Plex and Jellyfin, you set up a central server that will serve clients on all of your devices. So if you watch half of s2e3 on your tablet, then switch over to the living room TV, it will continue where you left off.

They also have clients for all major platforms and for web browsers, and can be accessed from outside of your home. I'm currently in Portugal, watching TV shows off my home machine in Germany via my phone or my Amazon Fire TV or my laptop.

It's much like having your own self-hosted Netflix.

I'd say it comes down to how diverse your clients and your media library are. Plex can transcode your content on the server and then stream it to clients, which means the clients don't necessarily have to be powerful. Kodi is more of a file browser, it just opens the content from whatever storage and plays it directly.

Kodi is (can be) client for Plex, so those are orthogonal pieces of software.

You can run the client on an existing Roku STB instead of building a separate box.

Because preferences. I use Emby and Kodi and generally I prefer Emby's UI.