In Dominions, magic boosters are important for giving your mages the ability to cast certain higher level spells and rituals. You may also simply empower mages and boost a magic path in exchange for magic gems, however this is usually less cost effective than crafting magic booster items instead.
Magic booster items only increase a mage's level in a magic path if they are at least natively level 1 in that path already.
A decent guide detailing the various magic paths and what boosters exist for each can be read here.