What is the purpose of Slender symbols?
The Tribe Twelve symbol is used to stalk and Marble Hornets symbol is used to either summon the Slender Man or receive protection from him. The Slender Man does not need symbols to be summoned or to see through as he has control over physics, the Observer does. But what about the theory that the Marble Hornets symbol is used for protection? Well the symbol appears in places that seem to be indulged in the presence of the Slender Man. At the beginning of Tribe Twelve, Noah goes up to the watch tower and you can see the Operator symbol. Later on Slender Man appears in the watch tower when Noah was far away. This disproves that the Operator symbol serves no purpose of power to the Slender Man, but might be meant to indicate his territory and as a mere result of insanity from runners.