It's a common phrase used to express:
* Suggestion or invitation: "Let's go to the beach! Por qué no?" (Why not?)
* Agreement or acceptance: "Do you want to join me for dinner? Por qué no!" (Why not!)
* Challenge or questioning: "Why don't you try it? Por qué no?" (Why not?)
It's a versatile phrase that can be used in various contexts, depending on the tone and situation.