To put it simply, Lifprasir serves as not a catalyst for spells, but amplifies them using the familar's life force as fuel. However, that is only half of it power and the the most gentle. It other half is the power to stockpile magic, spells, make them stronger and for lack of better word turn themselves into a bomb. Saito became Lífþrasir when Tiffania summoned him as her familiar.