It’s as applicable to marketing as anything else: pay peanuts, get monkeys. Or moneys, if you’re using a spellcheck instead of paying proofreaders.
What message does it send about your professionalism if your customers have to proofread your signs for you? (You’d think Sainsbury’s could afford to pay a proofreader. They’re not exactly expensive.)
And what message does it send about your products if your menus end up with typos?