It can destroy their credibility. It makes the business seem less professional and as if they do not know how to run a business. Misspellings and grammar errors are something that are easy to fix, so when people fail to fix them it makes them look sloppy and amateur.
Because in any workplace it is important to come off as professional and correct. It also is looked at as a good skill to have in the work place. Businesses look to higher people who the know check and re check their own work.