How is frequency chosen?
In the embedded arena, often a specific frequency is chosen because of constraints with the peripherals of the microcontroller.  For example, a 1.8432 MHz crystal (or multiple of this frequency such as 18.432 MHz) might be used because this base frequency divided by 16 results in a 115,200 baud rate for a UART.  32768 Hz is often used for low-power microcontroller applications because it is easily divided down to 1 Hz for time-keeping.

Here is a list of various crystal frequencies and the reason they exist.  The ones listed "UART clock" are often picked for microcontrollers for the reason given earlier; the specific one chosen depends on the circuitry of the BRG (baud rate generator) and desired baud rate(s).