The donut increases extractor pressure, and helps prevent spring kinking that will lead to breakage (took out a spring that had been snapped in half just the other day).

Its not necessary, but it does improve function MOST of the time. Combined with an extra power extractor spring, it can prevent full bolt closure because the extractor wont snap over.
However, the most common problem with ARs far and wide is failure to extract, so i put them in as a fail safe in all of my ARs.