Two reasons I mount a 4 stroke glow engine prop horizontal, the first is as Gary said ie deadstick landings. The other is because I store my planes nose down leaning against the wall and I like the engine to be at the beginning of the compression stroke when stored; easier on the valve springs that way. My 2 stroke glow engines get clocked the same way.
My 55cc DLE gasser gets clocked to 1:00 for hand starting reasons. Hopefully the day never comes when that 22x10 Zoar gets me. I use a paint roller to start the beast.