Why do boys cumm?
The biological reason for cumming (ejaculating sperm) is to impregnate a female. Of course you can also cum through masturbation and from having oral sex too but the biological reason is to breed and have children.
In the vernacular, the term cumm means ejaculate, also known as seminal fluid. 'Come' or 'cumm' is the pre-ejaculation liquid which comes from an erect penis before the semen does, or it can refer to the semen itself. The semen is the liquid with sperm in, though the pre-ejaculation liquid can contain some as well. Also, the thing that gets a female pregnant.