Can a City fan fill me in why this transfer makes sense for City?

I get that they need double cover but what does that mean? City has Sterling, Sané, Bernardo Silva and Roberts on the wing. I thought Sterling is pretty set as long as he keeps form and that he gets competition by Bernardo who is also very good. On the left, there is Sané and centrally, City has David Silva and KDB.

Considering this competition, where would City use Mahrez? Centrally in rotation with Silva and KDB? On the left as a backup to Sané - which would not really be Mahrez' natural position?

I guess there is an obvious answer stupid me does not see - 75M is a lot of money to pay for a player who's not an obvious first teamer, I'd say.