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I read that PLL are used in CPU to generate the clock, but I can't understand why.

I don't really have any guess of why this is.

Been there, done that.

Apart from other reasons mentioned here is a different one:

The marketing guys want to make the chip design as cheap as possible. Thus they prefer to use cheap crystals. The ones use for Ethernet fall in that category. So you often end up having to use a 25MHz crystal.

At the same time marketing want powerful processors. Thus the processor (Let's call it a LEG⁺⁺) should be able to run at 1 or 2 GHz.

The only way to do that is to use a PLL.

Or the processor can run at max 64MHz, but they want to have a USB interface which requires a 48MHz Clock. Again PLL to the rescue.

⁺⁺_{LEG is not a registered trademark. (At least as far as I know)}

PLLs can be used to multiply and divide frequencies. CPUs that have PLLs to generate their clocks are highly reconfigurable. The clock speed can be varied relative to the external reference clock over a wide range, and it's the PLL that makes this possible.

PLLs are used primarily to generate one or more faster or slower clocks from a reference clock.

You might have say a fixed 100MHz crystal, but then want to run your CPU at 2GHz, so a PLL is required to increase the frequency (a 2GHz clock crystal isn't feasible).

Additional you may want to be able to change your CPU frequency on the fly (e.g. a turbo clock). In which case you could have a reconfigurable PLL.

Furthermore you may need a different memory or peripheral clock to your CPU clock. Again a PLL and divider can be used to generate this from a single reference.

3 main reasons;

1) power savings for mobiles and extend CPU life keeping cool.

2) signal integrity is improved using xx multiplier for front side bus due to short wavelength, lower capacitance

3) flexible CPU speeds are possible with throttle on Clock multiplier and Vdd to allow burst CPU power and cool down.

Signal routing of 100MHz FSB is easy, > 1GHz is hard and your CPU is much higher, driver current increases with f and standing wave reflections distort square waves. While reducing CPU clock saves power and reduces temp.