Why the opening line is the whole letter
A hiring manager reading a cover letter has already read your resume. They are not looking for a summary of it. They are looking for a reason to keep reading, and that reason has to appear in the first sentence or two.
"I am writing to express my interest in the position of..." is the single most common opening and it communicates nothing. The reader learns only what they already knew from the fact that you applied.
A working opening leads with evidence. Name the most significant thing you have done that maps onto the biggest problem in the ad, and state it as a fact. This is why the tool asks you to paste the full job description: it needs to identify what the employer actually cares about before it can lead with the right evidence.
The middle paragraph is a mapping exercise
The body of the letter should read as a direct response to the requirements in the ad. Take the two or three requirements the employer emphasises most, and for each one give a concrete instance of you meeting it.
Use the ad's own vocabulary where it is a genuine match. If the ad says "stakeholder management" and you did that, use their phrase rather than a synonym. Many large employers run cover letters through the same keyword parsing as resumes.
Be honest where the match is partial. A letter that claims to meet every single requirement reads as fabricated, because no candidate meets every requirement. Naming one gap and explaining how you would close it is more persuasive than pretending the gap does not exist.
The company paragraph, without inventing anything
The weakest paragraph in most cover letters is the one about why the candidate wants to work for this particular company. It usually consists of a sentence about the company's mission that could have been written about any company.
If the job ad reveals something real, use it: a specific product, a stated challenge, a market they are entering. If the ad reveals nothing, keep this to one honest sentence rather than manufacturing enthusiasm you do not have. A short honest letter beats a long insincere one.
The gap analysis is the useful part
Each result includes a coverage table marking the ad's main requirements as covered, partially covered, or not addressed. This is not decoration. It shows you exactly where your application is weak before you send it, so you can either strengthen the letter, address the gap in the interview, or decide the role is not a good fit.
Most tools will tell you your letter is excellent. This one will tell you which requirement you have not answered.