I get dozens of spec CVs as well as replies to adverts.
A covering letter is essential. One page at most. It says why you want the job and why your experience and qualities make you the right choice. Give specifics which cover the key requirements of the job - "I can run accounts software from my two years with XYZ Ltd".
It is also a chance to explain any issues with your CV. I had one a couple of months ago which said "My A Level results were poor because a family member died in my final term, but you will see that I got a 2.1 Degree"; without that, hers would have gone in the bin.
Most important, it is your chance to show something of yourself as a person. These days most jobs are going to attract half a dozen applicants who are all perfectly capable of doing it and equally well qualified. Provided your CV is up to the mark, I want something to tell me "this could be a good person to work with, so let's get them along for an interview to see if I'm right". I like to see some evidence of a human being and a life outside work.
Short, sharp, relevant.