Requirement of pul down resistor of 10k for Input Shift Register (74HC165) after the input buttons
The inputs of CMOS logic parts (CD4000 series, 74HC, 74AC) are very high impedance, so if left unconnected may randomly appear as logic High or Low.  With the switch open, the resistor holds the inputs Low, but allows a closed switch to pull the input High.

If you want the inputs to be High by default, you would connect the resistors between the input pin and the positive supply, with the switches connected between the input pin and Ground.