Proofreading is often considered for things such as social science papers and English papers, but it is needed for other works of an academic nature. A proofreader with the right qualifications can work on almost any paper you have to write. Sure, it may not work too well for all your projects, such as advanced mathematics and programming, but most academic papers will benefit from a proofreader.
Take your essay project within a scientific field. Things such as marrying your results data and the expected and actual analysis, evaluation and conclusion are difficult. You may feel that you have not hit the nail on the head, often because there are holes in your overriding theory. A proofreader will pick up on this as an inconstancy and will be able to point it out to you. There are also numerous formatting and structure errors you can make within a project of this nature, and a good online proofreading service will point them out for you to amend, or amend his or her self.
Medical essays are particularly difficult and often have numerous facets to the point where a student may be confused by his or her own work. Here is a discipline where you have to narrow down multiple causes that tie with the numerous effects (symptoms) and then explain your choice for treatment of which there may be many. So many threads and facets within a paper may make the piece difficult to follow, and a proofreader can help fix that. A proofreader may also be able to format and fix your work so that it is written as typical medical texts are supposed to be written. You work may only be college level when you hand it over to your proofreader, but it may be journal level when he or she hands it back.
Do not lump together a proofreading service with just English and social science papers. They may be able to improve the quality of you work in your personal academic discipline.