I can sort of understand the one on the right. Without the resistor, the circuit would be shorted to ground and Vout spoiled.
But the one on the left... if light shines on the phototransistor, the circuit grounds out. If it doesn't, however, you get... some voltage less than Vcc on Vout. There's something very fundamental I don't get.
Remember, use small words. I had to google 'pull down resistor.'
And what makes the right one better? That Vout gets all Vcc, undiluted by a resistor?