An antenna-mounted amplifier (preamp) is used primarily to overcome the signal transmission loss in the coaxial cable between the antenna and the TV set. In areas distant from TV transmitters, overcoming this loss is necessary since the signals are already weak. In areas close to TV transmitters, a preamplifier is usually not needed, and can actually cause problems as it may cause the signal levels to be too high.
The only sure way to determine if a preamplifier is needed is to measure the received signal levels with a field strength meter. Since this is not usually available for most home installations, determining the correct preamplifier can be a challenge. One good way to determine it is to enter your address into the signal analysis online tool (such as the following, it is not associated to Shopper+/PrimeCables, but could be used as a useful online tool: https://www.tvfool.com/?option=com_wrapper&Itemid=29) and review the listing of available channels at your location. If the station you want to view is in the Red or Grey you most likely will see an improvement by installing a preamplifier.
Another possible need for a preamplifier will be on installations with long coaxial cable runs from the antenna to the TV set. If there is 50 feet or more of cable between the antenna and TV set, a preamplifier may provide an improvement.
One more situation that may benefit from use of a preamplifier is when multiple TV outlets are being fed from the same antenna. Since the splitter used to connect multiple TV sets cause signal loss, a preamplifier at the antenna can help overcome these losses by increasing the signal levels at the antenna.