To indicate their sexual identity.
Beards are present not only in male goats but also in females. However, males typically have significantly longer beards. This likely serves as a sign that signals to other individuals the sex, age, and physiological condition of the animal. Such signals play an important role in interindividual relationships.
An indirect confirmation of this theory is the habit of male goats rubbing their beards with secretions from their preputial glands, which are absent in females, containing sexual pheromones.
