This is a plumbing vent. All of the drains in your home have a P-trap immediately under them. When you look under a sink, this is the U-shaped pipe straight under the drain (it looks like a "P" if you tip it on its side). Each of these P-traps "traps" a chunk of water that totally blocks the drain pipe so that gases from the sewer cannot enter your home. Without a vent for every P-trap, water flushing through the drain system can create suction that pulls the water out of a P-trap somewhere in your home, where that empty P-trap will then allow sewer gases to enter your home.
Since these plumbing vents all tie into your drain system, rain water falling down a plumbing vent roof opening enters the drain system and harmlessly travels through to the sewer. Squirrels have been known to occasionally drop delicious treats down these pipes, so that and insects would be reasons to somehow protect a plumbing vent opening.
Your "turbines" cap off HVAC vents, where water falling down these vents wouldn't be so harmless.