PMs are usefull when you want to ask a particular poster some details without poluting or derailing the thread.

For instance, one person claimed that he could solo Dragua with Harlequin automaton, but wouldn't divulge how because he considered it 'his' secret. Now, he was most likely talking out of his posterior, or at best pre-buratinomore, but in case there was truth behind his words, some PUPs would like to know the 'secret'.