Your customers are your best marketers. No place understands this more than the museum.
First, museums know that people want to share their experience on social media. Both the National Gallery of Art and the Atlanta Aquarium (two places I've visited recently) provide wifi to speed up sharing.
After all, images and videos are free impressions.
Museums also understand product placement. All customers exit through the gift shop. Museums place the rainbow at the end of the road to monetize on your excitement. You don't buy the album before you listen to the song. See. Like. Buy.
The store is also a marketing tool – purchased products and gifts become artifacts of the lived experience. Some customers love the tour so much they donate and become annual members.
Exciting product and good experiences drive sales. The gift shop is icing on the cake.