mm wrote:

In a townhouse n'hood near mine, many of the houses have a pipe coming
out of the wall, and I wonder what it is.

It's a white plastic 4" right angle pipe connector that comes out
about 4 inches** and then turns to point down. Nothing is connected at
that point. It's at the same level as the garden faucet so I guess it
comes out of the ceiling of the basement.

**It only comes out as far as the right-angle piece puts it. There is
no other pipe showing.

....
Could also be a bathroom exhaust vent.
Maybe some other exhaust vent. Downflow cooktop vent?
Fireplace air intake?
I assume that you know that it is not a dryer vent?
If you have propane and your water heater or furnace is above
that level, it could be a propane "drain" (a safety "feature"
required by some building codes).
It could also be something that is no longer in use.