You use either of them after you have written something. It doesn't matter if it's just one sentence or one paragraph. You can still get it checked.
it helps by making sure that you do not misspell a word. also if you are typing something very important (document, speech etc.) it makes it look better by having all words spelt correct.
At the moment you cannot add any spellings to the WikiAnswers spell-check dictionary.
Online at whatever site you have your email service with.