Like Coyote said, it's so that the brake calliper can be swung out of the way when the wheel needs to be removed, same as a Lefty.I still don't understand the thinking towards using slotted IS fixings in the first place, as opposed to holes.

That way you don't have to take the bolts fully out, and drop all the washers/shims etc. in the grass...

Wheel removal instructions
Englund Cartridge Service
Fork Service Instructions
All the best,