People (not just Yidden, goyim too) often prefer books. Books are easier to read than screens and much more portable (they often fit in pockets, and never run out of power). They also have some books that very expensive, or hard to use in an online format (e.g. atlases).
Libraries also provide access to online resources that would be very expensive for individuals (ones that have paywalls and/or expensive subscription fees). They also provide public access to the internet for free (albeit with less privacy than a personal home computer, though that doesn’t affect Yidden so much since “of course” we never look at sites that require great privacy).