Pull-up resistance and pull-down resistance are resistances that stop the unstable state of the input and output terminals.
The input terminal becomes unstable state when the terminal is open and the connection destination is high impedance.
If there is a pull-up resistance and pull-down resistance, the state of the input and output terminals can be made to be High or Low even if the terminal is open.
For the output terminal, when the IC is not within the range of normal operation, such as during low input voltage, the state becomes unstable such that it is not possible to tell whether the output voltage will be High or Low.
When the output signal is unstable, the devices downstream from that input signal will enter an unintended state, which will cause an increase in the quiescent current or a malfunction. For this reason, the product must be designed to certainly be High or Low to prevent the devices from unintentional operation.
Further, in the unstable state, what the voltage will be cannot be known due to the IC’s leak current or the external noise, etc.
For this reason, the state may differ even in the same circuit due to the surrounding environment and variance in the IC or components.