For deals you get outside the MLS: wholesalers, agents, pocket listings. The deal shows as available at this price, whatever the MLS says, until a Redfin download shows it sold after the date you add it. Then it becomes Unavailable.
Keeps your sales database current: sold homes for comps, homes for sale, and listing changes on your deals. Each download is saved, so everyone with access sees the same data.
Rentals in a download are skipped for now; rental comps come later. Tip: after the first big pull, short windows keep it current with fewer files, for example homes sold in the last week and new for-sale listings for the areas you work.