When you go to a job interview you make sure your nailpolish isn’t chipped and that your clothes are clean, neat and appropriate. You should act exactly the same with a cover letter when you send your book to a publishing house or agent.
Proofread it. Polish it. Get a critical friend to check it for grammar, spelling and punctuation. Make sure you have done exactly what the publisher/agent asked you to do. If they said provide a one page summary of the book that means one page. And not one page in 20 point font or 6 point font either.
If they ask for a brief personal biography they mean a writing-related biography. List any publishing credits or experience you have. No matter how cute your kitten is, the publisher does not want to know about it.
KISS = Keep it short and simple.
If your cover letter is messy or irrelevant or does not provide the information they asked for, the chances of the agent/publisher reading your book diminish greatly.