- The employer told you not to
- They put together their application at 11:58 when the posting closes at 12 midnight
- They think their name or their mother's brother's uncle will get them an interview
- They can't write
There aren't many library jobs (ok, there are no library jobs) where you would never have to write. You have to verbalize everything in libraries and articulate directions. The cover letter is how you show them you can do that, so you get into the interview and demonstrate your speaking and persuasive abilities.
Ignore people who tell you not to worry about the cover letter--unless the employer has directly told you to leave it off for some reason.