I'm here to tell you that yes, yes you do!
You need an editor or at the very least someone to glance over your written thingy. Why? Because I see it all the time -- mistakes on websites, in articles, on resumes, on FB, etc., etc.
Sometimes it's just the wrong word altogether or a typo or the wrong use of a confusing word (is it affect or effect? advice or advise?) or an extra period at the end of sentence or missing comma in the sentence. Other times it's a bigger deal -- a sentence that isn't clear or a whole paragraph that doesn't get its point across. And sometimes it's all of the above!
Editors and/or proofreaders make you sound good. When your written copy sounds good, it makes you look good. And when you sound and look good, that's a good thing, right?
Good writing + good editing = good impression on others (bosses, friends, coworkers, and so on!)