a cover letter let's you give a bit more detailed information about your skills and ambitions than just a resume. It's also tailored to the store you're applying for in particular so demonstrates that you really want a job there and it's more than just a case of handing out your resume to any place that will take you. You can also show that you've put a bit of effort into researching the place etc.
Basically, a cover letter will make you stand out.