In many cases, a resume is the first (possibly only) contact with an
employer or hiring manager.
In the high-tech industry, submission of resumes by email and the web
even reduces the traditional cover letter to near-extinction. If you write
more than a couple of lines of introductory text in an email with an
attached resume, you're probably wasting both your time and the time of
your contact (which may actually be a resume scanning program).
Ideally, your resume should be targeted to a specific type of job.
Every company to which you send a resume should be receiving something
specifically targeted for them so that they can see how you can solve
their business problems.
This isn't always appropriate or even possible. For example, if you're just out of
college or you've not been working in the industry for awhile, you would
be better off demonstrating your well-roundedness.
If you're going to a job fair and plan on meeting with a large number
of companies, it's obviously impractical to rewrite your resume for every
employer, although you may want to have two or three variations so that
you can provide the most appropriate information.
If you're pounding the pavement and knocking on doors, the same
principle applies; provide the most appropriate resume for the company,
but you don't have to custom-build one for each place.