Here are some benefits:
-It's great for people who don't have a mountain of miles in their AAdvantage account, but have enough to cover mileage for one direction.
-You can opt to use miles for the direction in which airfare is high, then purchase a separate one-way ticket for the less expensive direction.
-You can redeem miles to fly into one city, and redeem miles to return from a different city.
(Example: San Jose-Dallas and Austin-San Jose).
-You can mix and match classes of service and book multi-segments on a particular trip.
Here is one drawback:
-There are fees associated with some award travel, which essentially could double if you book two separate one-way tickets. Those fees include close-in processing fees (booking award travel within 21 days of departure), and re-deposit fees should you decide to cancel your trip.