It's easy to miss problems of grammar, spelling, and syntax in your own work. Your proofreader is trained to catch the errors you miss and to find the errors that your computer's spellcheck cannot identify.
Regardless of how brilliant your argument is or how much time and energy you put into your research, grammar and spelling mistakes distract your reader and undermine your authority.
Having your work professionally proofread will give you peace of mind. You can hand in your work knowing that it is free from distracting grammar and spelling errors so that your ideas shine through!