Why do airplane wings have fins?

The fins cause the air molecules on the top of the wing to act in a certain way, meaning those molecules will not act in another way. This is a key understanding. Their random motion might push them down against the wing if they would be left alone, whereas the fins force their motion in a certain direction which means the down component becomes smaller, because a molecule can't be going two ways at the same time.

The molecules at the bottom of the wing are left more by themselves. They will naturally end up carrying upwards momentum to the wing because they move more randomly. So, it's not really just about the time the molecules spend on top of the wing, it's about controlling the randomness of the motions. I also dislike using "pressure" as this is an emergent phenomenon, and kind of complicates thinking about lift imo.

This is all a big simplification btw, there's several optimisations in place wrt shape of the wing, that do ultimately have to do with the time it takes for certain aerodynamics to take effect and remain, at certain speeds.

Putting small, tilted fins on the upper surface of a wing has the effect of stirring up the boundary layer of air flowing over the wing- mixing high velocity air from the outer portions of the boundary layer with the low velocity air that is right at the surface of the wing.

This energization of the boundary layer delays the onset of flow separation of the air flowing over the wing, a condition known as a stalled wing, which maximizes the wing's effectiveness in generating lift. This is particularly true when the wing is operating at high angles of attack.

By delaying the onset of a stall, the fins help the wing keep generating lift and the pilot's flight controls maintain their effectiveness at high AoA conditions.