The screen can be totally invisible if not in use. There is no need to reserve space for it. A screen can be mounted in front of a window, a door or a cabinet. (I have experienced all these mounting alternatives in different rooms.)
Just as Mike said. AUDIO is the other reason to own a projector and AT screen.Screen size, but also the whole presentation with nothing but image seen up front, since speakers and subs are not seen. They are hidden behind the AT screen and this ties the audio to the image very well. I like a big sound stage and that does not fit very well with a small screen.
I know what you mean about feeling like a kid again. A big projected image can really bring back some of the sensations of viewing a movie when you were young.There's nothing wrong with watching movies on TV,if its at least 65in,but like R Harkness said watching a movie on a projector is a different experience, being 63yrs old,it make me feel like a kid again!