They fundamentally alter (some might say "upgrade") your model of reality. They allow you to perceive things about the nature of reality and consciousness that you don't ordinarily have access to. For example, they allow you to perceive the unity and interconnectedness of the universe and all life on earth. They also allow you to perceive how illusory time is.
They will show you things about yourself you didn't know about, such as the reason you do certain things, or the reason you have certain attitudes and beliefs. They will show you that your ego is an illusion, and that the thing you call "you" doesn't really exist (because everything is infinitely permeable and separation is an illusion constructed by the brain).
They offer you an opportunity to fundamentally rewire your mind, by essentially dissolving the ego construct and rebuilding it from the ground up.
Imagine a path through the woods. If you tread that path long enough it becomes more and more ingrained so to speak. Eventually it becomes the only path you know. It's the only path you feel safe on. Psilocybin wipes it all clean and allows you to start creating new pathways through the forest. Not the best metaphor, but it's good enough for now. These things are notoriously difficult to describe without sounding insane.
So maybe you spent your whole life with repressed anger and feelings of inadequacy, but you didn't know it. Maybe you're not such a great person. Maybe you're selfish or not as good a friend as you should be. We hide so much about ourselves from ourselves in order to protect ourselves. That stuff will all come out during the trip. You'll see it clearly. That can be painful and frightening, but it's ultimately extremely therapeutic.
And the benefit of using psychedelics therapeutically, as opposed to traditional talking therapy, is that once the dose is down the hatch, there's nowhere to run. You might want to look away or make it stop, but there's nothing you can do. It will force you to confront the things you don't wan't to think about. And the harder you fight, the worse it will get. But if you surrender fully to what you are being shown, it can be the single most ecstatic and euphoric experience imaginable. But you have to let go.
Any questions?