Sure, sites like SeatGuru can show you which seats are the best—but in a 2D graphical form that doesn't really show you what the plane actually looks like inside and out. Aircraft Explorer contains 3D models you can peer inside of to check out what the next plane you catch will actually look like.
Like many of the apps currently available for the HoloLens, it doesn't do a whole lot. Right now you can just cycle through aircraft options and step inside. While there are several cool apps that are simply complex holograms, such as HoloPond, this one actually has a practical use. You might not need to peer inside the next aircraft you're going to fly on too often, but it's a nice benefit to have around when you want it.
HoloLens users can download it for free on the Windows Store.