One advantage of lower frequencies is that the signals have better penetration, it means they pass through objects such as walls with less attenuation.
This effect results in better in-building penetration. The primary advantage of lower frequencies, however, is that lower-frequency radio signals propagate farther in the environment.
At the lowest end are systems to communicate with submarines operating in the extremely low frequency (ELF) band which ranges from 3 Hz to 300 Hz. Wavelengths at those frequencies are thousands of kilometers long, so antennas, which have to be huge, are a slight challenge.
Mobile communications systems used globally have frequencies much higher than this, ranging from 450 MHz to 2500 GHz, with most systems in the U.S. operating at either 850 MHz (cellular band) or 1900 MHz (Personal Communications Systems) band. In trying to cover an area with the minimum number of sites, using 1900 MHz takes somewhere between 2 to 4 times as many sites as 850 MHz. The exact ratio depends on multiple factors such as path loss, the link budget, cell tower height, and the geometry of the area being covered. Lower frequencies, such the 700 MHz band in which LTE is being deployed in the U.S. right now, requires even fewer sites than 850 MHz, though only slightly.
