The free credits are designed to only enable you to play in as many leagues as you'd like, joining one league per day - when you purchase credits you aren't just allowing yourself to customize players and teams, but you're also paying to be able to join as many leagues as quickly as you wish and, more importantly, removing the ads. Once you use up your paid credits you will continue to be returned to the 5 credits daily so you can stay in the leagues you are in.