Why do you need HDRI if the area light covers the window?
Base on the title of your question I'm guessing you're talking about an area light in front of a window which gets then marked as a portal?
In that case the area light doesn't actually emit any light anymore. It works as a guide line so to speak to guide the light from the hdri through the window. It basically provides the render engine with a path to steer the light into the room more efficiently. You can reduce noise in interiors this way. Without the HDRI it would remain dark though.
I think of it this like saying: Here Cycles, I've got this portal for you, this is where I want you to throw more light through. I hope that makes sense. :)
