Why Pull-Up Resistor?

An often suggestion in Arduino debugging is "whether or not a pull-up resistor has been installed"

Why do we need a pull-up resistor? This article explains it well (and this one too), but in summary,

For an (digital) input pin, A pull-up resistor will set a default value when there is no signal at the input. In other words, when there is no input you don't want some random value but a definite value.

Also,

• If when there is input you measure 0 (or low), and when there is no input you want it to measure 1 (or high), then you must use a pull-up resistor.
• If when there is input and you measure 1 (or high), and when there is no input you want it to measure 0 (or low), then you must use a pull-down resistor.

But since Arduino has built-in pull-up resistors that can be enabled in software, you should think about a configuration that only requires pull-up resistor or you must implement the pull-down externally. However, if your external signal already has two definite states (high and low), then you don't need to enable the pull-up resistors inside the Atmel chip