The main purpose I could see for a "pop out floor" from the rear is rapid deployment of cargo in the back... the Gunship would be more than a troop transport I think. Versatility woudl be very important... Specializing the vehicle would limit its usefulness. This could be said for many SW vehicle designs... AT-AT's might have a variety of "fits" for their belly holds for instance, for carrying more than just soldiers into war.
It's the same argument that could made for the AT-AT having multiple means of deploying what is in its belly... A zip-line-only method of deploying soldiers limits that vehicle's abilities on the field, but I digress.
With the Gunship, I would see the ability to drop the entire floor out of the back as sort of a rapid deployment of equipment, more than men... A gunship hits the ground, or even just comes in low and drops the floor... Perhaps it's designed to slide a bit and the ship wouldn't even have to stop. Tied to it are cargo containers of medical supplies, munitions, food, etc.
Sort of akin to rapid deployment of cargo out the back of airplanes today... fly in low, and shove it out the back with small chutes to slow it down as it slides across the ground. There's everything from tanks to bundled cargo could be delivered that way.
I think that's the most plausible reason though... speed. It makes an interesting play feature on the toy though... That was one vehicle I think Hasbro got pretty right from top to bottom, as toys go. My only beef has always been its size... I'd like a new, larger one, for that reason. Including a lot of the same features would be cool too, like the deploying side doors, and the ability to lock the to-scale gun turrets on the sides. I'd dig both.
Having atmosphere and non-atmosphere doors would be a nice plus too.