Why should planners bother themselves about population?

Because overpopulation (with inadequate housing, utilities, and jobs per capita) can lead to the spread of disease, not to mention social issues such as poverty, drug abuse, and violent crime, which in turn encourage companies and educators to relocate to another area.

Under-population, so to speak, means that the city has sunk unnecessary infrastructure into a neighborhood that will not be filled to capacity (i.e. they've wasted money on the neighborhood). This can also provide an opportunity for lowbrow opportunists and squatters to move in, which can lead to the same social and economic ramifications as stated above.