What is it? Developing a thesis is a method used to create an effective thesis that has an argument, conclusions about your topic, relationships between pieces of evidence, and direction of where your essay is going.
Why do you use/do it? A thesis is used to show the reader how you will interpret the significance of the subject, to show the reader what to expect in your essay, make a claim others may dispute, directly answer a question asked, and your side or take on the subject at hand with presented arguments