Years ago, a colleague of mine had a company sign off on proofs, printing for a brochure - only the company name was wrong. They'd been signing off on proofs many steps along the way, the mistake never caught until it printed. I've been there, we all have - clients send you the wrong dates, the wrong spellings, expect you to fix their mistakes or just intuit their true meaning (sometimes whilst not paying you to do review/edit, just to 'do' .. eye roll). 

I read and reread my emails, my blog comments, almost anything. Typos are so hard in this fast, fast get it done in 2 minutes world. Harder to check your own work, even the obvious can slip past you. Harder to make sure your meaning is clear. Like anything else, at best you appear careless, sloppy when you mess up; at worst you miss a key opportunity, offend your best customer or lose that perfect hire. Then worse gets worse, when your dirty laundry goes public, goes social - damaging your brand reputation and you have to invest in damage control rather than building your biz. So, check - then check again. FWIW.