Many permit you to recommend books and other stuff for purchase or type in comments about what you like or don't like about the library. A few libraries are adding commercial products to their OPAC such as CD-ROMs of magazine and newspaper articles. So now you can do some of the research you used to do in the library from your home or office simply by connecting to the electronic catalog. Health Information CD-ROMs about diseases and treatments. Government Document CD-ROMs with the full text of government reports, census, legislation.
Libraries pay thousands of dollars to feature commercial databases on their catalogs.
So when you spot an OPAC database you'd like to use, have your library card handy and enter the registration number when you're asked.