First, the lowest frequency crystal that is commonly available is 32.768 KHz, called a "Watch crystal". Many PICs can use this as the primary clock, and can easily divide this frequency by an integer factor, or a factor which can be expressed as a ratio of two integers. If you use a much higher crystal frequency, say 8.000MHz, then the division factor can be much larger, and then the divided frequency can be controlled to a much higher resolution (smaller frequency steps).
The PIC can even count the divided frequency and display it on an LCD.