Can someone explain this
It’s capped
Is it suposed to be that way
well i can’t help you with an explanation, but i will say that, at least he used a good hubbless connector. could it be an old gas apliance vent that’s no longer needed?
A couple of years ago I did a partial inspection (I don’t normally do those, but this was a referral from a past Client whom I told to call me at any time with questions about anything) where the carbon monoxide detectors kept going off when they cooked, used the furnace or fireplace, took a hot shower, etc. No one could figure out why. This was an older house, so I wasn’t really expecting to be able to isolate the problem. However, after inspecting all the gas-fired appliances, I went out to look at the vent terminations. They were all plugged almost identically to that shown in the picture. I suggested that they remove the caps. No problems after that. I never did find out why someone capped them, but I suspect that someone had a little birdie get stuck in the kitchen exhaust vent (seen that many times) and decided that all the vents on the roof needed caps on them.
Is it possible that the vent was capped for testing purposes and neglected to remove after the test was complete?
Marcel 
I have one similar on my own house…except I went with the finest plastic bag and duct tape.
Mine was the result of my kitchen remodel. Bumped out, moved a stairwell and ended up with three sinks in various locations…all with Air Admittance Valves (AAVs). I “temporarily” capped the now cut-off vent to prevent rain in my new ceiling space. That was four years ago, and will finally get a new roof and remove the vent completely this year.
My point is, if I saw something like Earl’s, I would look below for modifications/remodelling and/or other venting methods. BTW…it also could be an old service mast…who know?
Not when it was installed and not if it’s being used for it’s original purpose. But it ain’t necessarily wrong now! Dig deeper.
As it needs a new rubber boot, I’d recommend removing it if I was certain it was no longer needed.
Looks like an abandoned plumbing vent, probably capped off because the bottom of the abandoned pipe is cut off and left open in the wall and would leak rain water into the wall if not capped.
Look for new PVC vents and then you know it was the old one.
Make sure you always report what you see, even if you do not have a recommendation. Observe and report…