Hats, clothes and shoes or "gear" basically give you stat ups depending on what they are and the more stars they posses the more slots they have, a one-star item has 2 slots, a 2 star item has 3 and a 3 star item has 4 by default, on lower tier gear you can use super sea snails to upgrade them or shuffle the sub-stats on them (the main one they have is not.)

Hope this helped. :>