The Home Theatre Projector has some handy features compared to the older technology of Giant Screen Plasma or rear projection systems. Cost alone is worth considering. A 'Big Screen', whether Plasma, LCD or Rear Projection can easily cost thousands of dollars, the larger, the higher cost. Even at 6 or 7 thousand dollars you may be restricted to 70 inch diagonal width.
A Home Theatre Projector System which projects images up to 120 inches is affordable at starting prices under $1500.
New generation of projectors are compact and light. New projector technologies such as LCD projectors or DLP projectors which utilize compact digital image chips which have brighter light output than CRT technologies. Now there's no need to completely see your movie in total darkness. A dimly lit room will do just as fine.

Remember having to arrange a party at your house for 'The Big Game' if you owned a 'Big Screen'. Now with a Projector, there's no need to rush somewhere, or the same spot every week, you can move the party around sort of sharing the load. Think of having the 'Super Bowl Party' or other event at a venue that can be picked by the size of the group invited instead of the location of the 'Big Screen'. You could even move to rented space if you expected a large enough showing.
Now imagine the same scenario with an online presentation, or Power Point Presentation. See, this is starting to show some possibilities.

You can setup on a table. It's also light enough that it could be ceiling mounted without extra bracing as long as you mount to an upper floor joist or rafter.

Don't tell anyone I said this, but this is also a possibility when you go on vacation, although if camping some may feel the 'Big Screen' is inappropriate. I know someone that might shoot me if I suggested camping with any form of TV.

A wide range of connectivity options is available. Imagine your PC display that large. Games become more engrossing or you can run your digital photos as the ultimate slide show.

For Display output you can use an actual projection screen or just a clean wall.

If you think about your normal existing Entertainment System, most of the hardware is in the front of the viewer. To have some of your equipment in a different location required cabling. Some surround installations run cables under the floor or carpet for speakers. You would think that possibly a projector would require something similar.
Nowadays though we have all in one home theatre with speakers and DVD player thrown in together in one unit. There are also wireless options, wireless projectors and wireless speaker systems. A wireless setup actually allows you to have most of the hardware in a completely different room from the 'Viewing Enclosure'.

Some have even gone to the point of building an acoustically designed, sound proof viewing room with theatre seating and the whole deal. Modern Hardware allows for the 'Theatre Experience' without the noise and sticky floors. All of this depends on your budget of course. My budget stops short of the acoustically perfect viewing room. You are allowed to make your own choices.