If it seems that it can't be possible that someone's broken leg is suddenly healed, there is just "no way" that you can accept that this is possible, and yet it happens in front of your face, or even to your own body, this is going to produce a whole bunch of reactions.
One of the things that witnessing a miracle will do is it will DEMONSTRATE to your mind, factually and experientially, that something has just happened which does not FIT into your belief system or your idea of reality. Something just occurred which defies all of the laws that you thought were real. That alone will EDUCATE you, by demonstration and experience, that the laws you thought were true cannot be.
It will also cause your mind to become momentarily aware of a state of being which transcends form. "Miracles are temporary shifts into invisibility, that is why they heal" - ACIM.
"Principle 16 - Miracles are teaching devices" - ACIM
If one moment your hand is burnt, and then in the next moment your hand is suddenly not burnt, this miracle will defy and transcend all physical laws. Your mind will not be able to process this at the ego level. It will be bypassed. Your mind will become aware even if just briefly, that bodies can't be real, or at least that physical form can't be 'stuck' being in a certain state, and this will give you access to TRUTH experientially. This will undermine possibly everything you believe in, but you won't be able to deny it. (The attempt to then DENY this 'attack by truth' will make you sick. ;-)
This is similarly what happened when Jesus went through the crucifixion. He did it to demonstrate that the body is not real and death is not real. When one moment the body seems to be being tortured and killed, and then seems to die, and then a few days later shows up again as though nothing happened... it's not just a matter of oh look, there's Jesus.. your mind instead is going to have an instantaneous shift in awareness.
If Jesus is showing you to your face, that bodies can be switched on and off at will, one moment being dead and the next moment being perfectly alive again, you aren't going to initially be able to comprehend HOW, and just the mere fact of you experiencing this will SHOW YOU and make you aware that the body CAN'T be real - that death CAN'T be real, because here is the PROOF of that staring you in the face.
This has a profound healing effect on the mind and CAUSES awakening.
This is why miracles are also teaching devices because they demonstrate the truth and bypass all of the ego's laws and give examples of, experiences of, the truth in action. They educate by example that the physical laws CANNOT be true. And so they have the potential to instantly awaken and to instantly destroy illusions and false beliefs.
If Jesus showed up next to you right now, and then told you to shoot him dead, and you did it, and then he got up and there was no bullet hole and he was perfectly fine, it would blow your mind. Your mind would HAVE to expand in order to accommodate the evidence. It would show you perhaps for the first time in your life a demonstration of how bodies are not real. And you would've been always assuming that bodies are fixed and objective and constant and all the other illusions about them. And now suddenly Jesus shows you that they are nothing but images which can be flipped into any state at a moment's notice... that totally blows your entire world view about what bodies are. It's highly educational.
That's why Jesus uses miracles for TEACHING, because they demonstrate the truth and teach by experiential example, which defies the ego's thought system and baffles onlookers, causing them to be forced into an awakening.
This is also why many Course students DO NOT REALLY BELIEVE IN MIRACLES, because it is such a world-defying, body-defying, mind-warping, totally transcendent experience that the ego cannot conceive of it, does not support it, doesn't believe in it, can't be open to it, and doesn't understand it. So there is a LOT of resistance against the idea that Jesus LITERALLY is teaching us to perform such miracles.