What is the purpose of a persuasive essay?

The purpose of a pursuasive essay is to sway your reader to your side of an argument. An argument can be on topics such as I think my parents should get me a phone for my birthday. You may here this argumet all of the time.

A weak reason could be, I am now in the eighth (or whatever grade your in) and i think im old enough to have a phone.

This would be a weak example because your saying your argument but not giving supporting details. If you want a strong, effective essay you want your reader to continue to read and know your whole thought process on the situation. To better an argument you should say:

I think I should get a phone for my birthday because I think I'm mature enough to use one now. My parents say the phones are bad for my eyes but they have one themselves. My mom knowns I go on the bus after soccer practice but what if i miss the bus? How will I get ahold of her if school is closed and nobodys there?

Hope this helps :)