I play quick matches just to finish my weekly challenges
PiggyHead I'm with you here. I like playing Quick Match and completing the challenges. 99.9% of the matches I play are quick matches. I've probably played less than 10 custom matches since the game was released. I like building up my stash of cookies because I can spend them on outfits in the shop.
Now they've started releasing Christmas outfits (and are allowing us to pay for them with cookies), from all of the quick matches and challenges I've completed I have enough cookies to immediately buy any outfit I like that can be paid for with cookies..
Plus, the weekly challenges give you XP rewards, which is useful in the early days for levelling up and getting more frames, badges, and outfits. All of this is important while you figure out which character/outfit you like the best. My favourite is the goose wearing the flight jacket, and weekly challenges helped me reach the level required to get that.