The role of the cover letter is often misunderstood.
The cover letter is a good vehicle to
- Demonstrate that you have done homework for this application. You have read up on the company and the specific role and you can intelligently talk about it. A cover letter that is cookie cutter and not specific for the role or company does more harm than good.
- You compare how your experiences in the resume stack up against the requirements from the job posting. Sometimes I have actually put this in a table: first column for requirements, second on how I meet (or don not meet) them. That's what most HR reps have to do anyway, and they'll like you more if you do the work for them. It's okay to have some holes in there (no one is perfect) but you should explain how do you plan to work around it
- Explain or comment on any "unusual" things on your resume: holesgaps, major changes, job hopping, whatever it may be
- Explain why you think you are good fit for the job and why you think the job is good fit for you.
The "tell us about yourself" question is more about your resume than it is about the cover letter.