We're actually not sure. Many ideas have been proposed including implantation of something through the eye socket. Or just being punched.
We have collected the photos as evidence that they are doing SOMEthing of significance to cult members.
Also look up left-hand path vs. right-hand path. Most are the left eye, signifying participation in the left-hand path.
We don't know if it's just ritual magic or something else/additional physical that they willingly undergo.
It's definitely a sign of participation in the cult, though. Note all the stars, politicians, celebrities.