Sigil Magic uses these glyphs as a means to bridge the gap between the conscious and subconscious mind. It compares in some respects to traditional "talisman magic", in which predetermined symbols (such as planetary or astrological ones) are used to embellish a physical device that will encompass the "power" represented by those symbols. Generally, the talisman is subsequently carried by the user as a "charm".
In contrast, a sigil is a customised tool designed to bring about a specific effect, and it's physical basis is only used once at the time of the subconscious implantation, after which it is generally (although not always) destroyed.
Sigils are monograms of thought, for the government of energy ... a mathematical means of symbolising desire and giving it form that has the virtue of preventing any thought and association on that particular desire (at the magical time), escaping the detection of the Ego, so that it does not restrain or attach such desire to its own transitory images, memories and worries, but allows it free passage to the sub-consciousness. --A.O.Spare, "The Book Of Pleasure"