Use proper grammar to be perceived as a person who is knowledgeable and educated. Using improper grammar sends the message that you are uneducated or not well-read.
In order to ensure that the message you send is the message received, use proper grammar. You risk confusing the message recipient. They may even ignore the message entirely if the grammar is poor.
Job opportunities often hinge on an individual's ability to speak and write effectively.
The use of improper grammar (such as slang) is offensive to some. As a result, the message and, ultimately, message sender are ignored.
Improper grammar on marketing materials can result in lost sales. Customers will not purchase products from a company that does not know how to sell itself properly.