Just to be clear (and use the right jargon), this is a soffit vent to allow more air into an attic. I added 6 of those on my old house. Earlier, I called them eave vents.
And this is an exhaust vent. My old house already had two of these, and I mounted an attic fan behind one of these, and avoided having to instal an attic fan through the roof. You might not have that option with a townhouse.