I'm not a master in this field. But I did some reasearch. I have got these explanations.

The Human brain put images together because they have learned to expect things, and sometimes the data might get a little confused.We may see an illusion because we know what we are expected to see, even though part of a picture or design may not be completely there. The basis of this is in how we perceive things.That almost explains everything right there. If our brain and eyes did not function like they do, we would not see illusions like we do.One example of optical illusion is the computer.Sometime when you have a chance, look at a computer screen really close for a minute or two. You will notice that you computer screen is made up of tiny red, green, and blue dots. The illusion is, you see more than just red, green, and blue dots; you see thousands of different colors. Our brains put the red, green, and blue dots together to make the colors.Optical illusions just trick you into seeing something else. They are an error in our perception of the illusion.