The cover letter is one of the central documents in your application. Here you have to skillfully put your qualifications and strength in the limelight for each job advertisement. The formal design of the cover letter already plays an important role and often decides whether or not to the person responsible for personnel.
When you write your application, it's a bit like a sales letter. They advertise for themselves. Therefore there are certain similarities.
Put yourself in the position of the recipient, usually the person responsible for human resources. He has to "pick" the best possible from a large number of applicants within a short time. So you have to convince with your cover letter by writing a plea for your strengths.