/sci/ - Science & Math

I have a stupid question about circuits. I was looking at a tutorial online for making a delay on circuit (this image).

So my understanding is that the voltage builds up against the capacitor until it reaches a critical value that overcomes the Zener diode. That then allows current to flow across the transistor, turning the light on. The delay comes from the time it takes for the capacitor to charge.

My question is: what does the second (the lower one in the image) resistor do? Why is this necessary?