Publishers and agents get a LOT of queries. So any extra information that can help them focus in on the most promising material is helpful to them. They aren't going to read everything, so they use these gatekeeper-asks to winnow down the slush piles. They aren't there to be fair, or to reward the deserving. They are running a business, and they are looking for a product they can sell.
Prior publishing credits are nearly always relevant, although the closer the past credit is to the current pitch, the better. Past results may not be an indicator of future success, but don't we all prefer to go with someone with a good track record? (With that said, past credits in a completely different genre may not help much at all.)
Biographies and resumes aren't often requested for FICTION, which indeed usually has to stand or fall on its own merits. But they are very relevant for NON-FICTION. They let the publisher know if you know what you're talking about or not. They also give some sense as to whether you have a solid platform or a preexisting audience. Those, in turn, indicate how well you'll be able to market your own work, which is a very big concern in today's market. Unlike fiction, which is almost always sold on completed manuscripts, non-fiction is often sold on speculation, which makes the track record even more crucial.
