In a professional environment, you are expected to be formal and act professionally. In personal communication exchanges (depending on the person), you have the freedom to express yourself a bit more informally (e.g. using slang, perhaps even profanity, etc.). If you had spoken in an informal manner in a professional environment, you may come across as unprofessional, perhaps even rude, impolite and inconsiderate. This could possibly put your job at stake in certain cases, and may even get you fired.
