I'm just gonna say it.
*puts on flame cloak*
Luck. At early levels, every three points you add to Luck, will reduce the cost of your spells. However, this only works when you are below being Master in said school(s). When reaching 100, this bonus goes away. There are many who argue that putting valuable points into Luck, for such a small benefit that goes away when you are level 100, is a waste. And they would be right if you will be a mage who uses the game's spells only.
However, if you venture into the custom spell area, you will find that the bonus for reducing the magicka cost of your CUSTOM SPELLS ONLY, to continue as long as you add points to Luck.
The percentage is vague, at least to me on console, but the benefit is quite real, and some characters create spells knowing that they won't be able to cast them unless their Luck is in the mid 60s. I'm usually perfectly willing to sacrifice those valuable Attribute points for more custom lightshow, at least for a "pure" mage.
*daddy quickly left*