Here's why:
* It answers the question "why?" It explains the reason for something (his lateness).
* It modifies the main clause. It provides additional information about the main action, usually a verb.
* It uses a subordinating conjunction. The word "why" introduces the clause and shows its connection to the main clause.
Example:
* He came late because he missed the bus.
* "He came late" is the main clause.
* "because he missed the bus" is the adverbial clause of reason, explaining why he came late.