Advantage is that if you have to change it's one change fee vs. two.

Disadvantage is that if the outbound is cancelled or ridiculously delayed you can't cancel/change the return.

If you have to entirely cancel the outbound portion of a round trip, AA (at least for me in the past, you can check on this) will allow you to retain the return, as opposed to the usual rule that if you miss one segment, the entire rest of the trip is cancelled.

I often have situations where I know when I want the outbound but am not yet sure of the return, so I book the outbound.