Discoverability in libraries is better, and difficult to replicate in an online setting I've found. When I found a book through the online database, I would go to get it and almost always the other books on the shelf or nearby shelves were also relevant to my research. But most of these books didn't show up in the results of my initial queries, so I would not have found them without the physical library.

Unfortunately, the libraries I went to, have drastically shrunk their accessible physical collections in favor of keeping most books in storage.

Big thing libraries also have is books that haven't been digitized. There are so many great works that simply cannot be found online.