Having worked in a supplier warehouse before, there are a lot of reasons for refurbished products. Here are the two most common:
Big-Box retailers are mindful of profit margins. As a result, when customers buy something, try it, then return it, it now is classified as a 'used' item. Big Box retailers tend to return these to the supplier for a full refund/replacement. The manufacturer cleans, repackages, and resells these through liquidator/wholesaler channels.
The other huge-volume refurb channel is when smaller retailers (regional/local) buy the items in lots, and the item is classified as 'sold' to that retailer as if they were the end consumer. If they go out of business, those items sometimes get sold back to the manufacturer in bulk. The manufacturer has to confirm the products are really in the boxes and resell them. Since they are often marked as 'sold', they can only resell them as used/refurbished devices (I can site a new egg-stremely painful experience where this occured - OEM wouldn't honor the 1 yr. warranty for an item since they listed it as sold to NE over 8 months before I bought it, and it died 6 mo. after I purchased it. 8+6 = 14. According to the manufacturer, it was 2 months expired.).