I'm not sure what kind of vent this is for, but I'm wondering if its supposed to have a cap to protect from rain.
Sorry for lack of description. I'm completely ignorant to whatever this is.
If it helps at all, I have two turbines on my house plus this pipe. So I know it's not for venting the attic. My guess is its for the bathroom vents maybe?
Almost certainly a vent for plumbing. It connects to the sewer, so it doesn't need a cap. It's highly likely there's a sink or other fixture nearly below it.
It's a vent pipe for the plumbing it connects eventually to the sewer.
It will catch a small amount of rain but the rain will stay inside the pipe and eventually reach the sewer.
You're probably not supposed to put rain into the sewer but this pipe catches such a small amount of rain that nobody cares.
Actually there is a better chance that it is a Radon discharge pipe roughed in at construction. That pipe is schedule 40 PVC. Nobody uses that pipe for sewer vents. Try to trace it down through the structure a d see where it goes.
They are plumbing Exhaust pipes
You need to leave the end open to allow the air to escape
Get your ear near to it, and ask someone to flush the toilet. if you hear sound of flush, then its what @Ack said in the comments (it prevents vacuum in your wastewater plumbing).
In my area plumbing vents don't have caps in the sense that they close the end of the pipe. But they do have lead flashing that goes over the pipe and is then worked in with the shingles or other roofing material. Finally a cap is installed on top. The cap is something like an upside down cup with a a sleeve that runs down the inside of the vent. See examples of these products here 1
In your case I would worry about UV/weather damage to the bare pipe. I'm also concerned about the seal between the roofing and the pipe. It looks like there's a black rubber seal but it's hard to tell in the picture.
It's a bathroom vent , connected directly to your plumbing . It does not need a cap to protect from rain however it could use a mesh metal cap to prevent critters from entering inside .
Not a clue where your house is, but in U.K. we attach an air admittance valve, which allows air in to aid draining of baths, toilets, etc. An open tube would do much the same job, but with a one way valve, there's no chance of it getting clogged eventually, or of any smells coming out. So much so, that they are often mounted inside the house, making the plumbing of them so much easier.
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.
