When you're driving in a car, you rely on signs to find your way around. The signs don't just appear when you've already arrived at a place or a junction, because if they did you'd just overshoot - the faster the speed-limit, the further ahead the signs are placed, to give you time to react and get yourself ready.
You need to think of the reader of your essay as very much like the driver of a fast car - they will be travelling through your essay at a much greater speed than you are in writing it. When you are learning to drive, you learn to read the road so you know what is coming up, and when writing your essay you need to design for a reader in order to allow for this kind of creative reading. The signposting you provide in essays is a series of cues which allows your reader to anticipate the essay's structure.
Signs on motorways have to be very large, because of the speed drivers are travelling. In the same way, you need to make the signposting in your essay stand out, in case your reader misses it: what will seem too obvious to you will not seem nearly so obvious to your reader. You can usefully put in a whole paragraph of signposting about every 500 words in your essay; these paragraphs don't need to give additional facts or contribute to your argument, but simply to remind your audience what the essay's goals are, and how the material in your essay is relevant to these goals.