What I look for in a covering letter is what interests the applicant about the job and about the company. There's nothing wrong with mentioning relevant experience here, but as Twyxz comments, in the covering letter this should be kept brief. I've read great covering letters where the applicant doesn't talk about themselves at all.
Assuming there isn't an HR department that's already binned half the applicants, you're trying to get the interest of the Hiring Manager, who is always going to have other things on their mind - they're running a department as well as recruiting. The covering letter is what makes them look at your CV/resume. The CV/resume is where they decide if the applicant is worth interviewing. The interview is where they get to know you. It's not until after the interview that they'll decide - usually from a shortlist of applicants all of whom could do the job - which applicant is the best fit.
But no one will ever be a perfect match for any job - even after years in position. Applicants often make the mistake of trying to sell themselves as perfect when they don't know what the Hiring Manager knows about the job itself, and about who else has applied. That never comes across well.