Different interface which gives you different looks/locations of buttons.
Different tools (one may give you hotkeys to skip forward/back 1,5,10,30 seconds, slow down the speed, etc.)
There are multiple formats for videos, one may not play all of them.
Some are free, some aren’t. Some have open source code (so you can edit it to your liking if you’re savvy) some don’t.
Some have a more responsive development team and identify/patch bugs in a more timely manner.