Personal proofreading is a little different to the proofreading you pay for. Personal proofreading is more than just catching your grammatical errors or checking for consistency. Personal proofreading is done to examine if you have said what you wanted to say. Has your message been put across in the manner that you desire?
Only you fully understand the points you are trying to make. The point of an essay is to get these points down in text so that other people can comprehend them in the way that you do. The trouble is that all forms of communication are subjective. People impose their own bias, beliefs and prejudices onto a piece of work and mold it into something that applies to them. And, that doesn’t even include the people that straight misunderstand your essay or misread it. Your job is to make it as clear as possible so that people have less room for personal interpretation, and by personally proofreading your work you can check to see if you achieved this.
Do people really impose their own bias, beliefs and prejudices onto a piece of work? It happens all the time. Take the bible for example. Jesus threw the moneylenders out of the temple. Some see this as a damnation of the Jews, some see it as a far-left wing rejection of capitalism, and conservatives see it as a sign that business and religion can live apart so long as they do not intermingle. Some people simply take it as a sign that business should be conducted in business locations (and not in temples).
This sort of interpretation needs to be avoided, and if you do not personally proofread your work, then how can you be sure you have not left gaping holes that are open for interpretation? You are Personal proofreading to check that you made your point, that you made it consistently and well. Once you have done that, then you can send it to a proofreader to check your grammar, syntax, etc.