I always tend to lie to myself. It's not that I'm stupid or something but very often I try to convince myself of something from which I know it's not true, just to avoid any kind of trouble.
But Tool is like the most honest band of the world, in every way, and they can make you see things so clearly! The lyrics combined with the deep going music which is able to touch you somewhere nothing else can, make you concentrate on the important things, it can bring up feelings from deep inside and your emotions ride rollercoaster. Not every song can achive this, but some do (e.g Right In Two).

They didn't change me in anything directly, but they often helped me find a solution in difficult life problems.
Besides, I think that the music they make is adaptable to your feelings, so you can listen to it whenever you feel like your emotions need to be "caressed".