Here's why:
* It describes a noun: It tells us about the size or duration of something.
* Example: "The longer road is the scenic one." ("Longer" describes the noun "road")
* It modifies a noun: It changes the meaning of the noun it's attached to.
* Example: "A longer wait" implies a wait that is of a greater duration than a shorter wait.
Important Note: "Longer" is the comparative form of the adjective "long". It compares the length of something to something else.