It is a code requirement to prevent the back-drafting of the odorous gasses from re-entering the hiome, via either other roof top vents or soffit intake vents.
I'm with Tump, you typically see this so odors don't get into the windows, or even the soffit intake of the overhang. I see them on flat roofs too, when there are RTU's on the roof in close proximity to the stink stack.
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