Alseid more or less got it. Coming from FFXI and with a lv.60 blm in ffxiv, you have some understanding about how precious MP is and how enmity works.

Overhealing makes it much harder for tanks to maintain enmity on their target and wastes your MP. It is generally much more preferable for the healer to either stand still and do nothing or use attack spells than to heal 2000 hp every time the tank takes 500 hp worth of damage.