Your essay is a journey through your argument or discussion. Your paragraphs are stepping stones in that journey. Signposts help to guide the reader through. They indicate what will happen, remind them of where they are at key points along the way, and indicate the direction your essay is going to head in next.
Signposting should happen in your introduction and throughout your whole essay. There should be some element of signposting in each paragraph. Signposting can be useful for linking your paragraphs together.
It also makes the reader feel as if you are in control of the structure.