What's the point of a cover letter when you put the same stuff on your resume?

For a cover letter you are supposed to put down your skills and why the employer should hire you.

But don't you do the same thing in your resume when you put down your qualifications?

True to a point...your resume states exact dates of employment and specific details of your qualifications and skills.

A cover letter should be a generalized introduction. Enough to get them to want to read your resume, which has more detailed information.

The cover letter should point out very clearly why you and only you are perfect for the specific job. Clearly point out the specific skills and goals you have that fit with the job. In the resume you are listing your skills, but in more general terms.

It is supposed to catch the eye of a potential employer. Very often if the cover letter is not enticing the employer will look no further....

You don't put your qualifications in a cover letter. The purpose of a cover letter is to introduce yourself and get the person's interest into reading your resume.

In addition to what the other posters said, the cover letter also shows if you have writing skills, if you know how to put together a business letter, if you can spell, etc.

No, you don't.
A resume is a list of experience, responsibilities and education. A cover letter is more personable. The letter introduces you to the potential employer, should include some personal information such as "I am a father of four ...." and should state your intent in applying for the job. It should include a request for an interview and a thank you for taking time to consider you for the position.

Cover letters are supposed to get the reader interested enough to read your resume. It also serves as a writing sample so that potential employers can see that you are able to put together nice, coherent sentences. It also serves another purpose. If there is any additional information that you want (or need) the employer to know that isn't included on your resume, you need to include it in a cover letter. Examples include: you will be relocating to the area in which the company is located, you are about to graduate college, who (seeing a familiar name in the cover letter may entice the reader to call you in for an interview) or what referred you to apply for the position, etc. This is also your chance to ASK FOR AN INTERVIEW.

A cover letter tells how you were raised and a resume shows what you have learned. Good Luck.