what i love about riot games is how money hungry they are, they legit ban you for 14 days but allow you to sign onto your account, you can't play but you're surely able to purchase fucking rp still and buy stuff can't even use the practice tool while banned which is idiotic as shit.
Because the only thing the Client has is the Store. /s
Sarcasm aside, there are other features that Riot probably would at least allow you to occupy yourself with; Profile, Collection, etc. - Collection in particular would generally be something they'd leave open, just in case in the 14-days you're banned you find some builds you want to try.
I'm not sure if Friends/Chat feature are still active while you're 14-day banned, but in the case that they are, they probably would expect players to want to tell their friends why they wouldn't be able to play for a while.