Reality and Illusions are two cities in the forest of sight. Their purpose is to convey sometimes it is much more easier and simpler than seeing things that are. That is the same reason why it is easier to imagine rather than seeing it yourself.
If we compare and contrast both the places than Illusions is just a thing that aren't there that we can see very evidently due to our mind imagining it whilst Reality is not any imagination, it is what that is present in front of our eyes. Illusions seems pleasant but Reality is often much more harsh.
It is said that Reality was just as beautiful as Illusions but due to the attitude of people of Reality towards their city, the city withered away. Their attitude was that they used to walk as fast as possible and looked at nothing but their shoes and as they walked faster and faster the city grew uglier and uglier. They did not pay any attention to the thing they just walked past by.