There is an excellent Wikipedia article on Flying wings so this page will just describe the aspects that are specific to models.
Flying wings have been modeled for a long time but the Zagi range made them very popular. Initially designed as slope soarers they were easily converted to electric power with brushed motors and nicads, and then later fitted with more power.
Aerodynamicists can argue over whether flying wings are more efficient and full size designers can worry about cargo volume and where to put the windows, but for modelers a flying wing has some huge advantages over a conventional design.
A hard landing with a conventional design usually breaks the prop, and possibly the gearbox or motor shaft. With a pusher flying wing, all this is protected at the rear. The battery is usually near the front but protected by an inch or two of foam. EPP is very tough in compression, and can be covered with tape to prevent it ripping. There aren't any long thin parts (like the tail of a conventional design) to break off and the thick wing section can be used to bury the servo horns. People have accidentally driven over their flying wings without causing any damage.
Ease of construction
With no fuselage or tail to make, a flying wing can be built very quickly. Typically the wings are cut from a block of foam with a hot wire, then glued together. Spars are glued into slots and holes cut for the servos and other gear. Corex is used for battery hatches and the wing tip fins, because it's tough and thin. Elevons are usually balsa, but can be Corex or EPP. Covering can be packing tape or low-temperature covering film.
These two points make flying wings very attractive to more adventurous pilots who aren't bothered about scale looks and don't want to spend too much time building. Flying wings are often used for combat flying.