A letter also does nothing to change the actual terms of an offer. Price, conditions, deposit and possession date are what a seller and their agent can legally act on, and a heartfelt story doesn't strengthen any of those on paper.
If you want to stand out, a clean offer with a genuine financing condition already arranged, a reasonable possession date and a deposit backed by a home inspection, typically $400 to $600, already booked does more work than a letter ever will.
Your agent's job is to make the numbers competitive, not to manage what a seller feels about who's buying. Get your hand-reviewed valuation from Joel Dyck.