Jfitter's explanation of the reason for aileron differential is correct for almost all situations. It's used to compensate for adverse yaw when ailerons are applied.

The reason you start adding aileron differential to different aerobatic aircraft is not necessarily due to some adverse yaw, but to compensate for a bunch of other adverse affects. Aircraft, because they must fly in air and support their weight against gravity have a large variety of asymmetries. You can't make a model act symmetrically about all axes at all times. Thus, the various ways to compensate for them.

Sometimes, you can use aileron differntial, sometimes you have to trim both ailerons up...or down...from the regular in-trail position, you have to add mixing to fix knife-edge behavior, alter thrust lines, move the canopy, and on and on and on. Remember how many flights it took to get your Pattern model dialed-in to just the way you needed it so that it would handle and present properly? That's working out all of the little asymmetries you wound up building into it in spite of the high level of care and craftsmanship you put into it.

All of this is why aircraft geared to a certain event tend to start looking alike. The top fliers start finding out which aircraft configuration best performs the given tasks...the designs start trending towards a certain optimum. When Pattern changed from the high-speed, one-maneuver-per-pass style to the early turnaround style, it didn't take very long before the models changed dramatically. Designed for one kind of flight, the old models just didn't do the new sequence particularly well.

It comes down to you do what you have to do to get the model to fly the way you want....just build it absolutely straight first...that eliminates a host of problems!

bax

(edited for typos)