Yeah, it mostly evaporates and only leaves a thin film behind. It's better than nothing if there's no lubricant in place, but will actually make things worse if there is a functional lubricant in place.

The WD in WD-40 stands for "water displacer." It makes water go somewhere else. Secondarily, it is a solvent, and it's great for dissolving glues, like the glue used to affix government-issued tax licenses to automobiles. It's not really a lubricant, but in a pinch it can temporarily function as one.

I like Swiss army knives, but they collect lint and gunk from my pockets. I use WD-40 to dissolve gunk, and to drive out water after an ultrasonic bath, but I lubricate with the light machine oil used for barber's clippers.