In addition to what everyone else has said about reduced reciprocating weight, another advantage of AR-style direct impingement is the straight line recoil. DI AR systems actually do have a piston, but instead of being separate and mounted to the barrel, it's formed by the space between the bolt and the bolt carrier (that's why the bolt has those gas rings). Combined with the bolt lugs, receiver extension (buffer tube) and straight comb stock, that means all the recoil force and reciprocating parts are all on the same axis directly back to the point of contact on the shoulder. This helps prevent the rising action created during recoil, especially during full auto fire. This also keeps the sights on target, allowing for quicker followup shots (and why the stock iron sights are so high above the bore axis).