Composing a lot of cover letters during a job search can be very challenging for job seekers. It's time consuming to write a letter specifically for every job for which you apply. So, it's not surprising that applicants often ask if they need to include a cover letter when it is not explicitly required by an employer.
The Simple Answer
The simplest answer is yes if you really want the job. A well-written cover letter will enable you to frame your candidacy for employers and draw their attention to your most viable qualifications. An effective letter will also making it clear that you are highly interested in the job.
If you are applying from a distance, your cover letter will enable you to present a rationale for relocation and/or to mention that you will be in the area shortly for a possible interview. Gaps in employment with reasonable explanations can also be addressed in your letter.
Employers often expect to receive cover letters even though they didn't actually list letters as a requirement in their job advertisements. Candidates who don't take the time to compose a letter are often viewed as less motivated for the job.
Keep it Short
Some job seekers are concerned about burdening busy recruiters with letters when they are not asked for. Make sure that your letters are concise (no more than one page with 3 - 5 paragraphs) and that every statement conveys something significant about your candidacy.
However, no letter may be better than a poorly written one. A well composed cover letter serves as a sample of your writing ability but, unfortunately, the opposite is also true.