Determine the concept of placing antenna on hieght
First, starting deployment simply placed cells in regions where there was high density of vehicle or human population. Once they were there, operator often did not want to go to trouble of moving them.
Second, antennas are classically placed on tall buildings or the mountains. Depending on accurate location of such structures, area covered by a cell may be irregular due to hindrance near the transmitter.
Third, some property owners or communities do not allow building a tower at the location where center of the cell falls. In such cases, directional antennas are positioned at a location not at cell center.