Why is there a buffer in an AR?

[MaineMan2]

Ian just uploaded a great video on Forgotten Weapons that answers this question. 
 

The most fascinating thing to me is that originally the bolt carrier was really long and included what is now a separate part - the buffer. This confirms my thoughts that it is the combined weight of the BCG and the buffer that matters. (Please ignore the optical distortion in the pic.)

[dpete]

...and its the buffer weight that is the easiest to change when you need more @ss to control the gas

[98Z5V]

The buffer is the dead-blow hammer that you need to thank yourself for, when you DON'T have bolt-bounce.