A euphemistic way of saying "cheap shit you put together yourself using proprietary nuts, bolts, screws, and a chinsy wrench that (hopefully) comes with the package." In most cases, the parts are designed in such a way that you are forced to use their ridiculous "tools," and in the event that you'll ever need to tighten down a nut again (and yes, you probably will, considering that the amount of torque necessary to actually tighten the nut down would destroy the "tool" before you ever got there) you'll have to keep the "tool" forever, with all of your other "ready to assemble wrenches."
I'd imagine that this euphemism was born due to the amount of cynicism towards its cousin, "some assembly required." Once 99% of the American populace was fully aware that "some assembly required" means "this product is as cheap as it is in part because instead of hiring sweat-shop labor to assemble it for us, we expect your lazy, self-indulgent ass to do it, while watching television, of course" manufacturers had to come up with a fun, friendly way of saying "take that guacamole-dripping, greasy burrito out of your gaping maw for ten minutes, uncross your eyes, get off your ass, and put it together your damned self, you stinky, hollow shell of a human being, you."
"Ready to assemble" is such a popular concept in the furniture industry that it has become an entire category of furniture in its own right. One can easily find "ready to assemble" entertainment centers, nightstands, dressers, coffee tables, etc, etc. The best reason to buy "ready to assemble" furniture is price, and the second is transportation. It's always easier to move a narrow box vs. a fully assembled piece of furniture. Theoretically, you can even disassemble the furniture in the event of a move to save space, and then reassemble everything when you reach your destination. This is, of course, assuming you saved the right tool for the job. The other assumption would be that the furniture is actually worth the time and effort it takes to disassemble, transport, and reassemble it. In most cases, "ready to assemble" furniture could just as well be referred to as "disposable."