Your cover letter should change every time you submit your resume to a new position. This is because your cover letter is an introduction to you, the first thing the reader of that resume sees, and a generic form letter will not give a good impression. It may even cause them to pass over the resume because they did not see you put any effort into the cover letter.
Think of it like this: what would you think of someone who introduced themselves to you using the same script they used on everybody else? Some things are worth repeating, like your name, but other things may not apply, and if that new acquaintance seems more into rattling off their accomplishments than starting a conversation about mutual interests, then you might smile politely and move on as quickly as possible.
In the same way, when you need an introduction, it should be personalized to the setting. Your cover letter can be tweaked if you have invested in one that has been professionally written, but it should always reflect the job you are applying for and the company who is seeking to fill the position. If you can have the name of the person reading the resumes, even better!
This reality is one reason a cover letter is offered in our A La Carte Services. You don’t necessarily need to rewrite your resume for every application, but it really is a good idea to put as much individualization into your cover letter as is appropriate. Just like a friendly introduction that makes you want to talk, a good cover letter makes them want to read your resume and bring you in for an interview.