It just gives more people the opportunity to see the auction. Not everyone checks for auctions every day or every week. Some only every 2 weeks or oncfe a month. It basically evens the playing field on who sees the auction and who doesn't.
It really does help. It costs a few hundred credits per month (like 300 i think) to hold auctions longer than 10 days, but I have found it to be well worth it! Kim