More important than presenting Sellers with an approval letter... is that the approval process helps you gain a meaningful understanding of how the approval will effect your real life affordability.
Sellers do, in fact, want to see approval letters from Buyers before "tying up" their property in an Escrow... and their listing agent will virtually always require it as a prerequisite for reviewing your offer... but ironically the letters are worth only the paper they are written on, since the underwriting process always digs up things the initial computer screening does not recognize.
Meantime, regardless of who needs it more (i.e. the Seller, or you...), it does take 2-3 days to do it right, so we are better off preparing this in advance. This way, if the right opportunity presents, we can take advantage without being held up.