The first seven overlays are mostly 50% grey, which means when you apply an "overlay" blending mode you only see the highlights. The other seven overlays have much more shadow depth.

The idea here is to play around with different blending modes such as overlay, multiply, soft light, etc. to see what looks best for your design. Combine several blending modes with the same overlay to fine tune the effect.

Additionally, you could even use these as a mask for exposure or color shift.

Check out the overlay pack here.