This could be a contender for the most trivial, pointless, boring and downright stupid thread ever but...
Does anyone know why virtually all garden sheds have the wooden lozenge shapes at each end of the apex?
Further to what Liam says, a diagonal cut of the same material as the barge boards gives the shape. As it is quite an acute angle pointing upwards, the side with the open grain does not take in too much rain, given the rubbish softwood that all but the most expensive sheds are made of.
Discourages large birds from sitting above your head as you go through the doorway.