The Operator Symbol

The Operator Symbol represents the Slender Man. It may or may not be a deterrent against him or it may attract his attention. Both cases are presented below, though none are necessarily mutually exclusive.

Repellant

 * The Symbol itself could represent the Slender Man's lack of face (a face crossed out), so as to block him from appearing.
 * It has been suggested that the Operator Symbol represents an eye, and is meant to trick the Slender Man into thinking he is being observed. Since he only Slenderwalks when unobserved, it serves that this may constitute some defence against Slender Man invading one's home.

Attractant

 * The Symbol is one that is put on trees when they are to be cut down. More generally, "X" is the traditional symbol that marks a spot that holds something of interest. Both of these may imply that drawing the Symbol marks one out more clearly as a target for Slender Man.
 * One theory put forth that it was just a symbol that made you think about the Slender Man, which made him more real.

It is possible that all of these theories tie together i.e. whilst the Symbol may temporarily block Slender Man as described, it may simultaneously increase his interest in the drawer and Slender Man's power, also as described above, making long-term use of the Symbol unwise to those he pursues.