Standard English is the formal English that you need to use when you write in coursework  or in the exam. Standard English is the form of English you learn in school. All written English should be standard -- that means it should be clear enough for anyone in Britain to understand it. Standard English developed as the main form of printed English in the 15th Century. At the time, every region of Britain used to spell words differently, but printers needed a fixed spelling.  Printers like Caxton chose the East Midlands dialect form which was used in London and the South East. Soon Standard English replaced all written dialect forms -- the other forms of English spoken around the country. It also replaced French and Latin in law and in academic work. In the 18th Century, people wrote dictionaries and grammar books which standardized spelling -- Dr Johnson's Dictionary of 1755 fixed many of the spellings we still use today. All written English should be Standard English -- any grammar rules you learn are for Standard English and you will definitely need to learn them to avoid making mistakes in your work. The rules of Standard English mean using the correct forms of words with the correct spellings.  Avoid slang words -- words that your teachers or friends wouldn't understand. You'll lose marks if the examiners can't understand what you say or write. Don't use dialect words. Every region has words or phrases that are only used there. Don't use them in your coursework, because you won't be understood. Make sure you revise grammar and punctuation you have learned, and learn the list of commonly misspelled words you have made. Cliches are ideas or sayings which have been used so often that they've become boring and unoriginal. Phrases like, "As good as it gets" "At the end of the day" "In the fullness of time" are all cliches. So are images like, "as fierce as a lion" "as cunning as a fox". If you use them you will sound boring and unimaginative -- that could mean you lose marks for writing and speaking style. So avoid cliches. Why do we need to use Standard English?
Because no one can understand dialect words. Because there are many kinds of English in the world, we need Standard English to make communication easier. Because local dialect belongs to certain region, not every one can understand it. Because standard English has been used for a long time.