You need to consider what a crystal in terms of electronics actually does. It oscillates at a frequency that is both known and remains relatively stable. The oscillation is at a frequency that may be relied upon as a master reference for accurate timing of events. By comparison RC and LC based reference oscillators can drift with temperature and are not normally considered an accurate reference where exact frequency is concerned.
In a microprocessor system where only one master oscillator is used and exact frequency is not critical, a simple reference oscillator is adequate. But where a microprocessor is generating signals that need to be compatible or synchronised with other sub systems that demand frequency accuracy, the crystal oscillator provides that required accuracy. Phase locked loop systems often use an accurate master oscillator to create other frequencies and these then have the same accuracy as the master oscillator. Consider baud rate generation, this can be referenced to a simple RC or crystal oscillator. To provide a known accurate baud rate, a crystal is normally used. A simple oscillator may be adequate but there is the element of doubt that is unacceptable in mission critical applications.
Where a reference oscillator needs to be of known reliable and relatively stable frequency, a crystal oscillator remains a relatively cheap solution that delivers excellent performance. If a drift prone LC or RC oscillator is acceptable in an application there is that option that is simpler and cheaper. The designer needs to consider the oscillator accuracy requirements of the system, including its interaction with other systems that may require accurately timed signals.
Fraser