What is the purpose of PLL in a general microcontroller
The PLL lets you be flexible with clock speed even after you've built the board, and of course, it lets you generate many different frequencies from the one onboard oscillator.
Honestly, just having a PLL so you can generate many frequencies off the onboard RC oscillator makes it worth having a PLL. That way you can operate flexibly with no external oscillator at all if you don't need one. From there, it's not too much more effort to make it so you can also re-route that PLL to an external oscillator.
The PLL lets you produce clocks faster than what is possible in a quartz crystal. Even though MEMS oscillators are available which can oscillate at much higher frequencies than quartz, you still might not want to operate directly off of one since a 400MHz external oscillator requires you to route a 400MHz trace.
As for how the PLL works. Do you know anything about music? Do you know how you can listen to a song and clap to the beat? You just keep equal timing between each clap and adjust the timing until each clap lands on a beat. Easy, right?
Now, do you know how you could do two, or even four claps per beat? A PLL does the same thing. You count your own claps and make sure the time between each clap is equal, but you adjust the time between claps until every fourth clap lands on the beat that you hear in the song, at which point you stop adjusting. In that way, you can produce a clap that is four times as fast even though the beat of the song is four time slower.