Best available lets a fan choose a price level and get the best seats in it, without clicking through a seat map to find them. They pick a tier, Opendate finds the strongest open seats in that tier, and the fan checks out.
For venues, this matters most on shows where the map is dense or the buyer is unfamiliar with the room, like a first-time visitor to a multi-floor venue. Fewer fans stall out staring at a chart, which means fewer abandoned carts and less time your box office spends explaining where the good seats are.