On a personal level, proper grammar is important to people's perception of how you communicate. From the way you write to the way you speak, improper grammar -- while sometimes appropriate in intensely personal or casual situations -- may convey a certain level of ignorance, even if you meant it to come across as otherwise. This may be true of emails or letters you write to friends or loved ones or oral conversations you have with associates.
Professionally, proper grammar is paramount in every form of communication. From the right use of punctuation in a business letter to the correct formatting of sentences in an email to speaking in proper sentences and diction, good grammar makes you look professional and organized. It isn't proper to speak or write in slang terms or use bad grammar in professional communications, no matter how relaxed your overall office environment may be.
Proper grammar is generally taught in both high school and college, giving students a foundation on the topic. As individuals get further into their careers, however, that foundation may diminish some, and those basic grammatical rules tend to be less automatic. Professionals can often avail themselves to learning extension or professional development courses throughout their careers to brush up on proper grammar rules and ways to better their various forms of communication.
According to the Virtual Vocations website, proper grammar does not necessarily mean that you should "speak like an English book." In other words, you need to find the right balance -- orally, that is -- between speaking with proper grammar and communicating on a normal level with others. For example, the site suggests using simple words when simple words will do; don't try to sound knowledgeable by using big words when you don't know what they mean. Avoid slang when possible and when it doubt, go with standard American English in your oral communication.