I think that outside of narrow engineering circles, most use of lubrication is based on a mixture of trial-and-error, folklore, and marketing. One reason is that most lubrication needs are actually quite low performance, and you could probably use practically anything. People use WD-40 because they have it around, and this adds to the list of its uses.

It's essentially a mixture of mineral spirits and oil. Used as a lubricant, the mineral spirits evaporate, leaving the oil behind. It might be enough oil to keep a mechanism working for a while, or it might not be.

It's a "water displacer." Oil displaces water, who knew?

It comes in a spray can, so you can get it into things like a bike shift lever. And you can get the over-spray on things like the garage floor.

Bicyclists tend to get really worked up about WD-40.