Q: What should I do if the job ad doesn't ask for a covering letter? Should I still bother to send one with my application?
A: Yes, you should always include a covering letter, even if the job posting doesn't ask for it. It's proper business etiquette, it helps sum things up for the hiring manager that quickly looks over your application, and most importantly it allows you another opportunity to sell yourself.
Q: Doesn't a cover letter simply repeat what's already in my resume ?
A: It would be a mistake to merely copy info already on your resume onto your cover letter. The cover letter gives you an opportunity to demonstrate your writing skills, to sum up important points, and to add extra info that is difficult to express on the resume.