Problem understanding non blocking delay via millis()

Projects Programming
2 
    if(digitalRead(pir) == HIGH){
     
      digitalWrite(lightOut, HIGH);
      unsigned long currentMillis = millis();
      if(currentMillis - previousMillis > interval){
        previousMillis = currentMillis;
      }
    }

What is the point of setting currentMillis and previousMillis? You never use them anywhere else.