For some people completing rings or seeing numbers go up is satisfying. For the same reason completing all quests in an open world game or leveling up a character in an MMO is satisfying and addictive to some people.
This is useful for those who don't find working out in itself to be a rewarding activity and may have a hard time sticking with it otherwise.
I used to track my hiking miles back in the day. All it did was justify poor eating habits or give me a false sense of pride.