Formal and informal writing is two different ways to talk or to write to people like: “go away”, that isn’t formal talk, when you say ” could you please leave” it is formal talk.
When you meet an elder person or a perrson your not familiar with you use formal langage, your showing respect, manners and kindnest to the person your speaking with. When you speak informaly you use it with a close friend, or your sibling.
They are two completly different ways of speaking.