The short version is you don't have to do this, but we cut out our floor, started with no drywall behind the bookcases, and wired for electrical outlets that would be in the bottoms of the bookcases.
I'm going to assume you already have a wall you're building these up against. (We didn't, which is why you see through the studs in the pictures — that was a new wall as we were building the frame of the room at the same time).
You will want to at least remove the baseboard on your wall so everything can fit flush against it.
IF you want to wire the cabinets for electricity (we put outlets in three of the bottom cabinets to plug in things like my laminator, pencil sharpener and electronic devices, and added three sconces above the bookcases), you should remove most of the drywall so you can get electricity in it. (I'm not going to counsel you here on how to wire stuff up — my official position is that you should hire an electrician for the parts of this project that could electrocute you).
The reason we cut out the floor is that we might replace the flooring someday, and Ryan said we should do it this way to make that an easier process down the road. You can choose to believe him or not!
But yeah, at this point you are committed, and you are straight up destroying your house!