Russian Polite Imperative

Did you know that we sometimes use the imperfective imperative instead of the perfective one (for example, садитесь and not сядьте) to show politeness?

For example,

Садись, пожалуйста! Sit down, please.

Проходите, пожалуйста. Please, come in.

Наливайте напитки! Help yourself to drinks.