Like in AoW - they require upkeep of multiple mana per turn and provide an effect on the whole map or through your controlled domain.  Can also sometimes be counteracted by other (expensive) global spells or enchantments.  For more examples, see this post.  Missing these types of spells mean the world will also feel less magical and vibrant and living.  It's hard to have a living magical world without global or domain spells that can affect the whole or your own part of the world, no?