Grammatical mistakes are still important to fully convey an idea.
Grammar is the foundation of language. 
Grammar plays an integral part of how most people communicate. Without it, our sentence would make no sense. Studying grammar and employing it in writing can be a difficult and tedious task at times, but our lives are much easier with it than without it.
Even a slight change in grammar can seriously alter the meaning of a sentence. 
For example, "an ice cream float made with fries, homemade chili, and cheddar cheese" is understood as separate menu items, but without the commas it might be considered a single one. As capable, competent writers, we need to be able to communicate our thoughts, feelings, and ideas without ambiguity.
Grammar will still have value after people graduate. 
Unlike the papers and homework assignments, the students will have to complete in their four years of college, using grammar doesn’t just stop. In fact, many people who are able to communicate with proper grammar already have a leg-up on their competition while job searching. Therefore, working on grammar is always a topic no matter how old you are and what job you do.