Again, I think it's a matter of exposure. When you post a new work sans auction, it appears in one daily email and on the "What's New" page for 24 hours (with about 200 others.) Then as newer art is added, its position moves back page by page, and it becomes one of 180,000 other paintings on the site. So it has increasingly less chance of being seen until and unless it comes up in someone's search (for genre, media, size, artist) or someone goes to your gallery page.
When a piece is auctioned, at some point it appears on multiple other pages: All Auctions, Just Added, Ending Soon, and, if you're lucky, DPW Picks and/or Active Bidding. Its position also drops back on some of those pages, but renewing the auction bumps it back to the top.
And yes: auctions are the original model for the "daily painting" phenom, which was established both for marketing and promotion. DPW originally developed not only so they could promote themselves as a group, but as a focused, one-stop, cost effective alternative to eBay. So I think it's likely some (or many) buyers look primarily at the auctions. I might go as far as to question why anyone would join DPW and not use the unlimited, free auction feature auction at least occasionally.
Other simultaneous exposure opportunities DPW provides are the weekly challenge, the Monthly Contest, and their Facebook picks of the day.