PLLs could be used to reduce the influence of noise ?

If your clock has noise on it, a PLL can filter it out. Remember that it's basically an oscillator that is being tuned to match its edges(through a feedback path) with your incoming edges. So if there is noise on your clock tree and there is no PLL, it might create a spurious edge in the device which could be disastrous, while a PLL might see that edge and make a slow modification to try and track it(depending on bandwidth settings) but it would basically filter it out. I have seen designs where that worked(although we still suggested going back and cleaning up the external clock).