When you write an essay there's a certain way that you're supposed to organize your ideas.
Essays typically consists of five paragraphs.
-The first paragraph is your introductory paragraph (you always indent the first paragraph). You just open up the essay with this paragraph, introduce your topic.
-When you're ready to build on an idea introduced in the introductory paragraph you indent. This becomes your second paragraph.
-When you touch on another idea you indent again, and this is your third paragraph.
-You do this again for your third paragraph; this is the last paragraph you use to get your final points of the persuasive essay across.
-The last, fifth paragraph is your closing. You indent when you begin wrapping up your ideas so that they connect with the overall theme of the essay.
An essay doesn't necessarily have to be five paragraphs, but you get the idea? As long as you have some organization. Every essay needs the following:
An introduction
A body
A conclusion paragraph
Indenting just helps organize these better.