you can only unlock a single chest for each champ each season.
so youre getting the chest for that champion. and your premade can only unlock the chest for the champion youre playing. this means you have to play 40 different champions each season at some point, or you wont get all the chests.
i dont see the point either. basically if you like to main a certain champ, you wont get a lot of chests.
what most people will do is play a couple of games in premades each week and hope they get carried on a champ theyve never played before.