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:

• Phoenix Power boosts Fire Magic & Fire Resistance.
• Summon Storm Power boosts Air Magic, but only under a Storm.
• Summon Water Power boosts Water Magic, but only underwater.
• Summon Earthpower boosts Earth Magic & Reinvigoration.
• Strength of Gaia boosts Nature Magic & provides some combat buffs, but requires more paths than the rest to cast.
• Light of the Northern Star boosts Astral Magic, but for everyone on the field, not just the caster. It also costs 2 Astral Pearls.
• Power of the Spheres boosts all Arcane paths that the caster has, but it also costs an Astral Pearl.

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.

Magic
Concepts	Blood Hunting • Communions • Empowerment • Gems • Items • Magic Sites • Research
Categories	Combat Magic • Ritual Magic • Notable Spells
Paths	Fire • Air • Water • Earth • Astral • Death • Nature • Blood • Holy
Schools	Conjuration • Alteration • Evocation • Construction • Enchantment • Thaumaturgy • Blood • Divine