Why Write Correctly?

Although some may not believe it, we English teachers don’t stress “writing correctly” just because we’re a little compulsive. There are all kinds of reasons—social, semantic, otherwise—to use standard American English, even as there are sometimes good reasons to speak in dialect or use slang. Why do you think it is important to write correctly? How does grammatically and mechanically correct writing help you and/or the reader? Or, from another perspective, are there ever times when it’s appropriate to write “incorrectly”? What might those be and how do they compare to times it’s better to dot your i’s and cross your t’s (as the old saying goes)?