Readers expect words to be correctly spelled. When reading, people read as if the words are correct, and will take the mispelled meaning as the literal word the author has intended. A misspelled or misused word will slow readers down as they try to impose meaning on the sentence given the word as presented. This reading expectation is especially true for readers who are reading in a second or third language instead of in their primary language. Readers may extrapolate errors in the content of a document if they see errors in the format of a document.
Readers consider correct spelling a sign of an educated writer.
If you have trouble proofing your own text, trade papers with a classmate and proof each other's work. Find a grammar handbook you like and use it. Make sure your subjects and verbs match in number. Try to keep nouns concrete and verbs active. Make the story you are telling interesting, not boring.