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.