Global Enchantment questions

[Targa]

Hi all. I searched the forum but didn't see an answer to this question. Why cast the global enchantments that are harmful? Why research them? I forget their names off-hand, but there's one that causes global sloth...income is reduced worldwide, etc...and another one that causes death or disease or something. Anyways...why would I want to reduce my own income and kill my own followers and cause disease and unrest in every province? These spells don't say "Will not affect provinces owned by the caster". Can someone explain the use/uses of these? For that matter, the "good" ones too...sure, it'd be nice to increase productivity and cash...but why would I want to do it globally and help my enemies?