You're a good writer. Maybe other people find your writing so strong that they ask you for help with their writing. Maybe you can spot a typo at twenty paces.
Or maybe you feel confident in knowing that your word processor's spellcheck function is backing you up. After all, it checks grammar too, right?
Think again! Everyone needs a copyeditor or a proofreader. After we review the same text time and time again, it's difficult for our minds to pick up errors. In fact, the mind fills in what we expect to see and want to have in our text, so missing or juxtaposed letters and words don't stand out to us.
Moreover, we all have our grammatical and style glitches. Another reader can help us seek out and destroy these blind spots, and that reader isn't your word processor. Typically, my spell and grammar check adds more problems than it resolves. It wants to add and detract commas everywhere and capitalize too many words. Don't even get me started on how it responds when I try to use a colon or semicolon.
Don't believe me? Think your writing is perfect? Send me a paragraph of your writing. I'll mark it up at no cost or obligation to you.