Origins

Female
Masturbation in females is a tool to regulate the conditions in the vagina, cervix and uterus, which is used to either increase or decrease the chances of conception from intercourse, depending on the timing of the masturbation. This timing is a subconscious decision. If she has intercourse with more than one male, it favors the chances of one or the other male's sperm reaching her egg.
During orgasm, the woman's cervix extends and retracts at each contraction (cervical tenting), and the opening to the cervix gapes open. If a seminal pool is still present in the vagina when she masturbates, a significant number of sperm will be sucked up into her cervix.

Male
The function of masturbation is to flush out old sperm with low motility from the male's genital tract. The next ejaculate contains more fresh sperm, which has higher chances of achieving conception during intercourse. If more than one male is having intercourse with a female, the sperm with the highest motility will compete more effectively.