[A_Brandybuck]

In my opinion, Sauron used the Orodruin first as a psychological weapon. A mountain, which spread fire, ashes, darkness was surely a uncertainty for the people of Gondor. It is the only one in the known world and always connected with the evil and with the darkness of the Enemy.

And maybe he prepared his 'assault of darkness', which came just before the Battle of the Pelennor with these eruptions.