There are several important reasons to have a frequency that is accurate and stable for the broadcaster that means business about getting every yard of range possible. Real Estate systems typically have a PLL (phase lock loop) frequency base system. This type of system is stable enough for real estate sales, or a toy. A real estate transmitter system typically has a +-30 ppm stability at room temperature. That means that your frequency can vary by +- 50 hertz, or a total of 100 hertz. If the unit is installed outside with an extreme temperature range then your frequency could be off by 100 hertz or more due to the temperature affecting the PLL accuracy. Also with some PLL circuits, heavy modulation can cause the frequency to shift.

This extreme variance of frequency makes  these systems useless for multiple transmitter applications. If you have 2 units operation even 30 hertz from each other, you will hear the difference tone. (You will hear the 30 hertz tone, a low hum).
 

Also a reason the big stations stay right on frequency is beat notes with other stations. If you are operating 80 hertz off the intended frequency and there is a distant full power station operating at the correct frequency, listeners will hear a difference tone along with your audio, bad news.

Part 15 transmitters need to make use of every legal trick possible. One of these tricks (or methods) is to be right dead on frequency. Any distant receiver trying to lock onto your signal or pick you up will be looking for your signal on the correct frequency. If you are off frequency significantly then the receiver will have trouble locking to you, also your signal will not be in the center of the radios filters so your range will suffer.

All of Rangemaster systems are crystal based for extreme stability, no PLL here. RangeMaster systems have as good or better stability then high power transmitters designed for licensed stations.

See these links    quality  why