Differential pairs are used in high-frequency data lines, which are most prominent in devices used in sophisticated applications. And sophisticated devices tend to have associated literature to assist in the design where they describe which traces need to be routed as differential pairs.

Unless the signals you are dealin gwith are in excess of several hundred MHz, it does not need to worry too much about the intricacies of differential pairs.

Except for the comments and answers, there are a class of analogue applications where we use a differential interface.

This is usually where we have very low level signals, such as from a microphone, thermocouple or perhaps a strain gauge.