You do not always need to include a cover letter, especially if the online job you are applying for tells you not to submit one. Otherwise, you should still plan on submitting a cover letter with your resume.
The key here is that it must be done properly. In short, make it good and make sure that it is in line with current accepted standards and best practices. (I have included some links to some helpful videos providing guidance below.)
Do it right or do not do it at all is the key takeaway here. Including a bad cover letter is arguably worse than not including one at all.
Solid rule of thumb: If you really want the job, include a cover letter and take the time to make it a good one. Like all other factors in a job search, taking the time to do this and do it right will help separate you from your competition.
With respect to the cover letter, you can expect that roughly 50% of candidates will not even take the time to submit one. This is a controllable that you should work to your advantage. Do not let laziness keep your dream job from you.
Additional Arrow in Quiver
Think of the cover letter as an additional asset that you can utilize to help explain parts of your background that may not be easily translatable via resume alone.
Ensuring that your resume is in tip-top shape and aligned well with your LinkedIn profile, including a lean, mean and clean cover letter that provides additional context about you and the value you will bring to an organization can serve as the needed boost to get you over the hump of other candidates you will be competing against.
Please see the links below for videos we believe are helpful.