The first use of por is for talking about the cause of something. You can think about the translation of the English phrases ‘due to’ or ‘owing to’.
English: The office is closed for vacation.
Español: La oficina está cerrada por vacaciones.
English: We arrived two hours late due to traffic.
Español: Llegamos dos horas más tarde por el tráfico.
English: They cancelled the concert due to the rain.
Español: Se suspendió el concierto por la lluvia.