When would be occasions to use a lower clock given that high speed would always be preferable?
The microcontroller draws more power when running at higher clock speeds. In battery-powered devices, running at a lower speed may improve battery life.

Additionally, many peripherals derive their clocks from core clocks; the requirements of those peripherals and the devices they're connected to may place requirements on core clock speeds. (To give a specific example: The USB peripherals on certain STM32 microcontrollers require that a 48 MHz clock be available. If the core clock is not a multiple of 48 MHz, this isn't possible.)