When readers and writers don't use the same format — the same code — for applying capital letters and punctuation marks, confusion often results. Using the rules of the code enables you and your reader to understand the same things. Take a look at the following:

when the envelope arrived i opened it and screamed this is it i yelled in a voice that was loud enough to wake up the whole neighborhood running up from the basement my husband asked whats wrong nothings wrong i hastened to reply weve just become the latest winners in the state sweepstakes now well have enough money to go on that vacation weve dreamed about

At this point you've probably given up trying to decipher what's being said. Obviously, the words are jumbled together without any capitalization or punctuation, so reading them requires both time and trouble on your part.

However, if the story is rewritten and uses appropriate capital letters and punctuation marks, then it's a snap to read.

When the envelope arrived, I opened it and screamed. “This is it!” I yelled in a voice that was loud enough to wake up the whole neighborhood.

Running up from the basement, my husband asked, “What's wrong?”

“Nothing's wrong,” I hastened to reply. “We've just become the latest winners in the state sweepstakes. Now we'll have enough money to go on that vacation we've dreamed about.”

Much better, wouldn't you say? The same words are used, but now you can easily read and understand the story because capital letters and punctuation marks have been correctly inserted.