The purpose of a dissertation is to close your academic chapter. It's a complex and lengthy research paper in which you develop a multi-faceted subject.
It has an introduction for presenting the main issue and setting the context. Then, you start adding the existing studies. Next, you explain your findings and methods and end with a lengthy discussion.
The conclusion serves to present the solution to the problem while touching on different sections of the dissertation.
It must be original, both in the execution and concept.