A cover letter is used to convey the job seeker's interest in the position for which they are applying and highlight specific aspects of the job seekers qualifications that align with the requirements of the position. Whether or not to include a cover letter with a job application will forever be a debate, with both perspectives, to include one or not to include one, being valid.
The primary reasoning behind including a cover letter when applying for a job is simply that a cover letter was requested by the hiring manager in the application instructions. Not including a cover letter when one is explicitly requested would essentially be submitting an "incomplete" application. Most recruiters and hiring managers will only review "complete" applications.
The debate comes into play when the application instructions do not explicitly state that a cover letter is required. So, in those cases, job seekers must use their best judgement on whether to include a cover letter with their job applications.
For example: if a job seeker is applying for a job that is considered a lateral career move, where the new job for which they are applying is very similar to their current job or most recent job, and the job seeker is only in search of a change in job environment, a cover letter is likely not necessary.
However, if a job seeker is applying for a job that is somewhat different from their current or most recent job, in job level and/or job function, including a cover letter may be beneficial. In those cases, a job seeker can use the cover letter to explain to the recruiter and hiring manager specifically why they are interested in this type of career move, and also detail how their experience and education makes them qualified for the position.