I never understood it but what is the purpose or motive behind 'facials' in pornographic films? Or basically an adult male ejaculating upon the face of another individual most cases a woman's face?

It's called the "money shot." It proves that the man did cum and how much. Peter North is well-known for prodigous amounts of sperm. A facial is a type of male dominance because most women do not like cum on their face, hence men do like it. Similar to anal; most women do not like it, so men do. The more degrading to the woman the more the men like it, for the most part. This can extend to cum-swallowing and cum-swapping between two female partners. To carry it further is the pissing and scat acts, which I'll not get into.