Why is grammar and mechanics important?

I believe grammar and mechanics are important when it comes to writing essays and other formal works. It is important because people need to make a habit of writing properly so when they are applying for a job or a college no "slang" is used. Also, proper grammar is important for writing an everyday essay or even a blog. If one practices using proper grammar and mechanics more often in their daily lives their writing will become better and will contain less fatal flaws.