Why do some Arduino-compatible boards provide a crystal for the CH340?
This is because some varieties of CH340 (namely, CH340B, CH340C and CH340E) have a built-in oscillator and cannot be connected to a quartz. Compare the pinouts of G and C chips for example:



Since the two chips are identical except for the oscillator part, it makes sense to design a single PCB compatible with both ICs, and simply leave the quartz circuit unpopulated when the CH340C is used.