Grammar or punctuation mistakes distract the eye. When you have commas thrown around willy-nilly, it is hard to focus on the topic at hand. Random periods and fragmented sentences also distract the reader. The key to any type of writing is to make it flow and to not distract the reader from the topic at hand.
What happens when there are lots of mistakes in your writing?
1. You lose credibility.
2. Your reader may not get your message.
3. Your resume might get trashed.
Now, none of those are things you want to happen. To prevent them, just proofread. Proofreading can save you from an embarrassing moment. Trust me.