YC : takes 7% for 125K and then let's say you raise $1m at $12m post money. Total diluted 15%.
Non YC : If you want to raise $1m for same 15% dilution - your post money would be $6.5m.
So these are pure numbers[1]
With YC, I guess Founders' gain a network, bay area connections etc
[1] : https://docs.google.com/spreadsheets/d/1kxOBF0CPhcktmgFvAZtY...
It's a natural process, and there is no such thing as not being biased (as it is a bias in itself), but the signaling of being part of YC is definitely huge, especially first time, and for outside of the Bay.
Investors are people too.
Angel list had a page where one could find out the valuation based on various filters. Unable to find that now. However, many YC companies fail to raise in demo day! And there are non-yc companies who 've gone to raise $3m-$5m in seed alone with just a deck. The latter had the advantage of proven track records etc. Also, the higer valuations at seed stage can come back to hurt in Series-A.