When you express yourself in writing, spelling or grammar mistakes can be a distraction from your credibility. If you're a blogger or professional writer, your credibility immediately takes a hit compared to a writer who knows the basic tenets of the language.
Effective communication is integral in a business situation. Strong communication skills involves clarity through proper grammar. This is a skill which helps to maintain a business's reputation professionalism.
Grammar helps to increase accuracy. When you study grammar, you become more accurate in that particular language. If someone is unable to write properly, it affects the effectiveness with which they express themselves.