Why these libraries welcome the bat colonies that live among the books

In Portugal, there are two 18th century libraries where colonies of bats are invited to roam free. Why? They eat the insects that would otherwise munch on the pages of the books shelved there. From Smithsonian:

In Coimbra, a colony of Common pipistrelle bats makes their home behind the bookshelves of the university's Joanina Library, emerging at nightfall to consume flies and gnats and other pests before swooping out the library windows and across the hilltop college town in search of water....

Whether the flittermice took up residence here 300 years ago, when the library was built, or more recently is unknown. Librarians do know they've been here since at least the 19th century; they still use fabric made from animal skin, imported from Imperial Russia, to cover the original 18th-century tables, protecting them from scat left by the library's flying residents. And every morning, just as their forebears did, the librarians remove the skins and clean the library floors.