I would imagine that it’s important to elect both presidents and congressman that best matches our morals. Although a signature of the president isn’t necessary (although recommended) he sure can make it difficult for them. A law/bill takes sometime to write up and if it’s vetoed, it can take up to another year to pass it through.
By then, some congressman can be out of office. So although they don’t need the president, he certainly can slow it down and inevitably stopping it by the mere change of congressman.
I can only see myself voting for a pro-choice president if he was the lesser of two evils.