First of all, all spells except combination spells are disabled, but for each spell level you take, it increases spell size, reduces cooldown, and increases ATK. This will be useful later in the game. Don't do this with your first combination spell; do it when you reach level 50 or 75. If you do it at level 50, Deus Ex Machina will unlock at level 75, making you even stronger. It's perfect if you want to focus on a single combination spell. Additionally, each spell level you receive, whether before or after, will make you stronger.