Air Conditioning Condensation Line
You will not see this on newer homes but older homes have PVC
condensation lines for air conditioners. The moisture in the air
conditioning unit leaks off of the evaporator coils and down to the
bottom pan where it is collected and then drained outside of the house
through the PVC pipe.
These pipes do not necessarily have to be PVC. If you see a copper pipe that looks like this it could be a hot water pressure relief drain. Keep this in mind when inspecting a house.
Newer houses drain their condensation into one of the plumbing traps underneath one of the kitchen or bathroom sinks.
You can see the condensation line here is not sealed properly between the stucco and the PVC pipe. A little caulking would do the trick. The drain in the future could also stain the green wood post holding up the patio. Dry rot could be a problem here also eventually, depending on the usage of the air conditioner and of course moisture evaporation on the exterior.