Cash and Diamonds are used in order to purchase garments to dress your model. 

Garments that can be purchased with Cash will be garments that are currently in-season and available solely through the in-game Shop. Cash can also be used to reset the Unworn Bonus for a garment that has already been used.
Diamonds can be used to purchase previous season garments, and to purchase current season garments that were released as prizes. Diamonds can also be converted into Cash or Tickets.