Summons powers and beings from other worlds. Many spells in this school will bring you additional units. –Illwinter
True to its name, the Conjuration school has the most Summoning spells, in Combat & Ritual form. It's not the only school with summons, however; for example, Death nations will get cheaper and potentially-more-useful summons out of Enchantment.
Conjuration also contains path-boosting spells that are incredibly useful in combat, at least when they can be used:
If you don't set Research goals, then all of your Research points will go to Conjuration. This isn't too bad, since Conjuration contains Summoning Rituals that are important for almost all nations, but neglecting the other schools is an easy way to fall behind later on.