how to use attachinterrupt instead to increase and decrease brightness

const int brightnessUp = 2; // pushbutton 1
const int brightnessDown = 3; // pushbutton 2
const int ledPin = 11; // LED 3
const int maxBrightness = 12; //# of steps

int brightness = maxBrightness; int interval=1;

void setup() { pinMode(ledPin, OUTPUT);
pinMode(brightnessUp, INPUT);
pinMode(brightnessDown, INPUT);
Serial.begin(9600); }

void loop(){
 if (digitalRead(brightnessUp) == HIGH && brightness < maxBrightness){
 brightness = brightness + interval;
 Serial.println(brightness);
 }
 if (digitalRead(brightnessDown) == HIGH && brightness > 0){
 brightness = brightness - interval;
 Serial.println(brightness);
 }
 delay(100);
 analogWrite(ledPin, map(brightness, 0, maxBrightness, 0, 255));

}

And the point of adding interrupts to your program is what?

Paul