Punctuation is the glue that holds together a manuscript, or any other form of writing. Without it, many sentences would be difficult to understand. You wouldn't know where you were supposed to stop. You wouldn't be able to tell the difference between statements and questions. Needless to say, correct punctuation is very important. When you were writing your rough draft, punctuation probably fell by the wayside. Now, as you're revising, it's time to place those commas and periods where they belong.