The most basic ones are a timer so you can see how long you’ve been going and how long you have to go, and also track streaks if you fast days in a row for motivation. They keep adding bells and whistles though.
I use app only for the sole purpose of tracking my fasts. Just seeing those streaks going up everyday, makes my day. Plus, I tend to get quite hard on myself sometimes, so seeing those fast hours getting too extreme, I know it’s time to slow down a little.
Some people are also interested in getting some extra info, join challenges etc.