Why A beam antenna is a directional antenna: it uses multiple elements (driven element plus reflectors and directors, as in a Yagi) to concentrate radiated power in one favored direction instead of spreading it evenly all around. That concentration is what produces gain, since the power that no longer goes off the back and sides shows up as extra signal off the front. Receiving works the same way, so a beam also reduces noise and interference coming from other directions.
Watch out The omnidirectional choice is the opposite of a beam by definition, and the name has nothing to do with a person or with aluminum structural beams; the word refers to the beam of radiation, like a flashlight beam.
Think flashlight: a beam antenna points the energy one way, trading coverage behind you for gain ahead.