twitter audit allows only one free twitter audit per account. If you want to re audit your twitter account you need to buy there pro account. But pro account holders can re-audit others account. so i think you are talking about those services?
So all they do is check how many of my followers are real? I thought it was kinda auto follower or booster who cares about spam amount
Some companies require an audit of 80% and a person wants to check how many of his live subscribers. That's why they provide such services