There are quite a few essay formats and styles. They are created in order to build a uniform bulk of knowledge in different fields. It would become a lot harder to do research in a certain field if every essay you looked at was referenced, formatted and styled in a different way. If they are all done in a similar fashion, then research becomes less of a stop-start endeavor. Each discipline will tend to have its own format and style, though many disciplines share the same type.
Why don’t they all share the same style and format?
The reason is because different disciplines have different amounts of data to convey, and what works well for one discipline such as philosophy, may not work so well for biomedical science.
Why do the rules keep changing?
If you look up a style such as Chicago, APA, etc, you will find a few books on the subject. These books are updated on an almost yearly basis because the rules keep changing. This is to help accommodate how the disciplines that use them have evolved. There are also other factors, such as how some styles have adapted for changes in technology, and some have simply been improved to make them a little more user friendly.
This is used for things such as English papers and philosophy papers, but you may find it used in numerous essays where conveying a lot of written information without causing confusion is key. The aim is to give page numbers and such when giving quotes–instead of just referencing the entire work. This helps a researcher find snippets of information quickly if he or she wishes to fact-check a piece of work.
This is the American Psychological Association, so as you may expect, it is used within a lot of social science papers. As odd as it sounds, this formatting appears to be set up so that it is more difficult for a reader to lose track of the theme of a paper (hence the running head), and to make reading the text as a whole a little easier (hence the double spacing).
This is the Chicago manual style, and is often used when working on history papers. It is a great formatting and quotation style to use if you are going to focus heavily on numerous sources. Footnotes are used because having to check facts against just a very large bibliography may be too time consuming.
This is often a go-to referencing style in colleges and universities because it is widely known and will suitably cover a lot of academic papers in college/university. In the real academic world, it may come up a little short with very long projects, but since there are fewer long projects in college/university the Harvard system sits very well.
A side note
Referencing styles and formats are well documented to the point where there are plenty of books on the subject. But, your college or university may have its own variation, or what is known as their preferred style/method, so do check with your college/university for the best formatting/styling resources.