If you are into emoticons, you can use custom emoticons of one server on another. If you share files, such as clips, until recently discord allowed only 8mb on free tier. Now it is 25mb, but can get up to 500mb with nitro. Video streaming is limited to 1080p at 30fps, nitro unlocks up to 4k with 60fps.
Basically allows more bandwidth and storage on their servers, which is understandable I think.
I have the basic $3 one, I just call it my shitposting tax. Longer messages, using emotes in any server and larger files. I use it a lot since my friends haven't warmed up to Matrix yet, might as well support them while its not total trash
I mostly just pay for it because I've used it for ages and definitely get more value out of it than the subscription cost. I don't think I use a single nitro feature.
There's a config file setting where you can make discord allow itself to be run no matter how out of date it is but it's hard to remember, hard to find every time you forget it and I don't remember what it is.