When roofs are pitched (i.e. angled), the snow collects differently than it does on flat roofs. Unbalanced snow loads occur on pitched (most commonly gable) roofs, and the building code tells us to consider these loads up to 7V:12H roof pitches.
Here, some of our engineers are at Loon, mid-mountain, and this building has quite an unbalanced snow load. The wind blows the snow from the windward (left) side of the roof to the leeward (right) side. The roof acts as a wind screen, so the snow stays put on the right. While this may be an extreme location at higher elevation and with man-made snow production nearby, it illustrates why we design for higher unbalanced loads in the right conditions.