They do it because most, well some ARTF aircraft just dont come with flaps. 90% have Ailerons! Hense Flaperons i presume?

Flaps work better than flaperons. With extreme flaperon down, your lateral control goes away, and in fact reverses.
Splitting full span ailerons into seperate surfaces; ailerons and flaps is a better choice.

About 30% of the semi-span becomes a flap while the aileron response is hardly affected.
On a Cub, with barn door ailerons, flaps work extremely well.