Purple Tickets are a type of reward in Bingo Pop - you can redeem them for Pets at the Ticket Exchange Machine.
Look for the purple Tickets icon along the bottom left-corner of the Lobby to access the Pet Collection screen. From there, tap on the "Exchange" icon in the top-right corner to cxchange your Tickets for Pets! Each Pet will be part of a Pet Set, and completing and leveling up your Pet Sets will provide some great rewards! Additionally, earn rewards by reaching the max level of each Pet.