From Arvid on Utreon:
"Why does an AR-15 need a buffer thingy? Why can't it just have a spring like every other normal gun?"

The Ar-15 really doesn't need the buffer and tube, but it is a holdover from the origins of the system: the AR-10. The intent of the AR-10 was to create a 7.62x51mm battle rifle that was very lightweight (under 7 pounds, originally) but still soft-shooting and controllable. In order to do that, Eugene Stoner. has to pull out all sorts of tricks. As it applied to our question today, this included a straight-line design with a buffer on the end of the bolt carrier to absorb any residual impact of the bolt carrier on the end of the receiver tube. At this time, there was no apparent need to allow for a folding stock, so the bolt was allowed to run the full length of the stock to minimize felt recoil.

After the basic design was put in place, the disassembly was changed from sliding together to pivoting, and this required splitting the single very long bolt carrier into what we now recognize as the bolt carrier and the buffer. When the design was scaled down to the AR-15, the basic architecture stayed the same, even though the recoil-reducing elements were not really necessary in the new smaller cartridge.