Most people like to take away a tangible reminder of their visit to a museum or famous place, whether it's just a postcard or a guidebook, and 'official' gift shops tend to be fairly priced and the goods on sale of good quality. Replicas for example are now of such a high quality that they're hardly distinguishable from the originals (I don't mean full size replicas of Stonehenge :-) but the sort of replicas on sale at places like the British Museum gift shop (the Lewis Chess Set springs to mind). Interestingly, it was the Conservation Dept at the BM that pioneered the very high quality of replicas you can now buy at the gift shop there.

There's quite a lot of other stuff going on behind the scenes at the big museum gift shops - the BM has its own publications department and not only publishes exhibition catalogues and books written by the curatorial staff but also prepares information packs for school children, gallery lectures, tours abroad etc.

At the end of the day the visitor doesn't have to buy anything - but it's nice to have to option.