Project 2 - question for understanding

I have 2 question to understanding why things are the way they are:

  • why do i have the 10k-ohm resistor added? the book says:

You'll also ned to add a 10k-ohm resistor from ground to the switch pin that connects to the Arduino. That pull-down resistor connects the pin to the ground when the switch is open, so it reads LOW when there is no voltage coming in through the switch.

can't i just connect without the resistor to ground? or even just leave it out, since it is connected to pin 2 as INPUT?

  • why are the resistors for the LED's just before GROUND, instead of right after the pin? e.g. right after coming from

pin 3 => 220-ohm resistor => LED => GROUND

(similar to the circuit on project 1 where it was

5V => 220-ohm resistor => push button => LED => GROUND

Thank you in advance for any hints.