If you don't put quotation marks readers wont know the characters are talking to each other, I can affect how the reader reads the essay. Or when you don't put a period the sentence wont sound right, It can also affect how they read it. With grammar and spelling, if the word isn't spelled right then the reader can read the story wrong.
In some cases it can even set a different tone! Lets say in the book flowers for Algernon they showed his intelligence decrease and increase by using bad grammer to excellent grammer!
