I thought the glyph was the source of power, not the spell itself. The spells are more like recipies, with glyphs as ingredients. Otherwise it's like saying tomatoes exist for the pizza, or eggs for pancakes. In reality, pizza and pancakes use things that can be used for something completely different. In fact, the egg is (or is supposed to be) a baby chicken. Instead, we use it for pancakes and house decorations during a certain month of fun.
That's just my take though.