What is it for if not for lubricating things? I've asked this question to many people and they never had a good response. Most people secretely just use it as a shitty lubricant.
I used to use it as a carburettor aide for rarely used lawnmowers to kick start them into life but this is probably bad for the engine. There are real products for this.
Penetrative oil for mildly rusted on screws and bolts.
And.. a really bad chain lube, or hinge de-squeeler. Basically these things deserve proper lube molybdenum/disulphide stuff. You're depending on the graphite (I think) in wd40 to do the job before the solvent strips all the real lube out.
In general if you need to lubricate anything important, you should look into what the official documents recommend. The right lubricant varies a lot depending on the application. Some stuff uses thin oils. Some uses sticky oils. Some wants grease. The right thing to use depends on things like manufacturing tolerances, the presence of oiling systems and the operating environment, so best to just check the manual.
Especially the second thing depends I guess on capillary properties, which other lubricants (e.g. silicon oil) don't necessarily have?
I've also used it on bike chains, to clean off a lot of the old nasty before reapplication of a purpose-created lube.
One might suggest:
A) Reading what it says on the tin can what it is for
B) Consulting its Wikipedia page, which states in the first sentence that it is a https://en.wikipedia.org/wiki/Penetrating_oil, and that article further explains what those are good at, and what not.
I use it to winterize my motorcycle in winter months giving the metal parts a thin film of WD-40 (being careful to avoid the brake calipers and rotor).