[Zenst]

Short version: It changes the refraction index and less photons bounce of in the direction of the observer, so it looks darker.

[k__]

How does it manage to direct the photons away from the observer?

I mean, it obviously doesn't know where they stand.

[Khoth]

The actual short version is: diffuse reflection from the surface means that some of the light leaving the surface is at angles which makes it get totally internally reflected at the boundary between the water film and air, giving it another chance to be absorbed by the surface

[skykooler]

It directs them into a more specific spot from which the material appears bright. This is why wet things can look "shiny".

[k__]

Thanks, that makes sense!