I've never heard of or used Roku before, from a quick search it seems to be a chromecast style stick to plug in to a TV to watch stuff, is there any reason you couldn't plug an RPi or your desktop straight into the TV to watch from?

Well roku's are popular, have nice software, and are easy to stream to. They are also newbie friendly, hand someone a remote and they'll likely have netflix, amazon prime video, youtube, and related streaming services going in no time. Even a 5 year old can manage to get things going.

It's the best I've seen, way better than I've seen built in TV's. It's also the Switzerland of streaming services. No problems with Apple devices disliking google services, or vice versa. So you get the best selection of streaming channels I've seen.

It's also popular enough that clients written for roku tend to be better than less popular devices, like say some random brand TV. Not to mention TVs are often useful for 10+ years, and it's way cheaper to upgrade a Roku than replace a TV.

Oh and there's a Plex client.

Rokus are streaming boxes/sticks that connect directly to streaming services (like Android TV). Next to the Amazon Fire TV devices, they're the smoothest way to get YouTube and other services on a TV.

My desktop is two floors away from either TV in my house, so being able to stream my personal media library to them over LAN is more convenient than lugging around a desktop to either TV. It's buttery smooth streaming too, and the interface is really customizable