The QP requirements become a lot harsher the more you advance into the game and the more servants you get.
Maxing one SSR servant costs 170 million QP while maxing one SR servant costs 85 million.
Double Bella Lisa and Mona Lisa give you 30% more QP, which is 10% more than just Double Mona Lisa.
That difference might not improve things too much for Main Quest/Free Quest progression but in Event farming and especially with Raids it is a world of difference when you are constantly farming nodes that give a minimum of 800k qp and over 2 million at max. During the upcoming Case Files and later the Summer 4 event those CEs can net you hundreds of millions of extra QP in the long run, which means you don’t have to spend time farming doors.
Also, the shop, sells 5 copies which is enough for MLB.