First, what you wrote are called Idioms. Second, idioms, slang, phrasal verbs are used(mainly) in informal speech to make it more natural and usual. Third, some of idioms and phrasals are used in formal, but I recommend to avoid doing that.

Hello Weiwei, Yes as Arnold mentioned it is not recommendable to use them in official letters or writings and if you do so then in moderation ,like including one idiom in a letter because using them often reflects sort of an intimacy that is not required in official correspondence.Moreover idioms are used often for stylistic embellishment, thus more appropriate to literature rather than formal writings. "Slang" on the other hand is to be totally avoided in formal writings.