I had to learn that the hard way myself.
My first character was a mage, back in the old 1st edition days.
At some point during a night during the storyline, an assasin climbed in through the window. Luckily I awoke. I tried to fry him with a spell (powerbolt, I think, it's been something over a decade, so bear with me if I'm a bit hazy on the details, yeah?).
Before he got to roll body, I'd still done more damage to myself (albeit stun) than to him with my spell. Why? I'd forgotten all about Spell Pool.
Somewhat later, having dispatched the assasin with my predator I (and possibly help from a few friends, not sure), we went into the Ork Underground, looking for a guy. They insisted that he not be harmed if we found him, so I took him down (when we did see him) with a sleep spell.
Remembering how my first spell had done pretty much nothing, this time I remembered my spell pool, and used it all. Sleep is (was) just a manabolt with 'Stun' damamge instead of physical. But I'd overpowered the spell enough that he took M physical.
Needless to say, we got him out, even though the Orks did frown upon the method.
So, basically, it's all about maximizing effect, while minimizing drain.
Which is why I stopped using Fireballs back when the 2nd edition came out.