What is the purpose in having a 180 degree elbow on top of a plumbing vent stack that comes out of the roof of a home?

to keep out the rain etc

To keep things from falling into the vent. It's not normally required.

That particular vent might have made a whistling sound when the wind hit it or because of your prevailing winds, it may have allowed air pressure to come into the house and make bubbling noises in the traps. That's really annoying. It might also be some sort of odor trap. some vents get gassy and putting a carbon filter on top of the vent stops that.

To stop birds perching on there as they are liable to become overcome by Sewage fumes and fall into the stack and cause a blockage,

Some birds will start to make a nest on the stack too.

An elbow is 90 degrees. I expect you mean two elbows or a U- section. This would stop birds nesting, perching & pooing at the end of the shaft, i.e. to maintain the air passage.

Air vent, and stop birds building nests in the air vent and blocking the vent.

Probably to prevent rain entering the outlet and snow blocking it.

To keep rain and birds from falling into it.