Here's why:
* Imperative sentences give commands or make requests.
* Periods are used to signal the end of a declarative sentence. Declarative sentences state facts or opinions.
* While an imperative sentence is a command, it's still a complete thought. Therefore, it needs a period to mark the end of the thought.
Example:
* Close the door. (Imperative sentence)
* The door is closed. (Declarative sentence)