Here's why:
* It connects a dependent clause to an independent clause: For example, "Whenever I see you, I smile." (The dependent clause is "Whenever I see you," and the independent clause is "I smile.")
* It shows a time relationship between the clauses: It specifically indicates that the action in the dependent clause happens any time the action in the independent clause occurs.
Other examples of subordinating conjunctions include: because, although, since, while, if, unless, etc.