A brief lesson in physics is needed. Don’t be scared. No math.
Think of an antenna as the launching point for radio waves. If the antenna is a single element, those radio waves will depart at equal levels in all directions. A car’s AM/FM whip antenna is an example of this.
Remember the days of rooftop TV antennas? They had many elements. That wasn’t just to make them more unsightly!
Antennas may also include reflective or directive elements or surfaces not connected to the transmitter or receiver, such as parasitic elements, parabolic reflectors or horns, which serve to direct the radio waves into a beam or other desired radiation pattern. – Wikipedia
That’s exactly what’s going on at the cell site.
Why bother sending signals where they’re wasted? With a directional antenna you can redirect power to where it’s needed. In most cases that means concentrating nearly all a cell tower’s power parallel or slightly down toward the ground. Pointing down slightly is necessary because of the Earth’s natural curve.
The extra gain in the antennas beams, used to increase signal strength to us on the ground, is taken from what would radiate upwards!