i think most of the peole who go to museums have library cards already.
But some do not. Far be it from me to dispute this with a librarian, but heck, life is short. YOLO!
I think that museum tickets are just one part of the mix that attract families to the library. Different groups are drawn with different bait, and once hooked they may find all the other offerings.
I use the library mostly for books and DVDs and online services. But I see how they offer a huge range of services including safe after-school space for kids, museum tickets, document scanners, 3D printers, English language learning for immigrants and more and more.
The quality and breadth and importance of my local library services continually astound me.