Question

In French why do they have masculine and feminine?

Answer 1

they serve to differentiate the kind.

the masculine is used for boys, but for an object or another
Exemple : UN garçon (a boy)- UN chat (masculin) (a cat) -UN pays (country)- UN ballon (balloon)
feminine for girls :
Exemple : UNE fille(a girl) - UNE chatte- UNE ville (a city)- UNE balle (a ball)

Answer 2

It is all based around a boy and a girl. If you had a girl cat then you could tell that it is a girl cat without the name or asking.