Well, I use our public library a lot (at least three of the librarians know me by name) but always for the same reason: books. I'll try to spread this out over ten semi-distinct concepts.
1) Books! Not even close to everything I want to read, but a nice selection.
2) It's free. Well, my tax dollars go to support it, but on a daily basis it doesn't cost me any money to use it.
3) I discover books I might not spot at the bookstore.
4) Audio books! I'm still getting into the habit of using these, but I really like the concept.
5) YA is a section I don't shop in but still love to read, and the library lets me feed that habit.
6) The children's librarians are awesome.
7) While I don't need to use the computers there, I recognize how valuable they are as a community resource.
8) DVDs! Although I rarely tap this resource, it did keep me sane the last time I was trapped at home with vomiting children. Their selection of children's videos is amazing.
9) There's a nice film series in our library's basement on Thursday evenings.
10) They run a small used-book section and are always happen to take my donations.