The difference of course is "intention". If you are simply using Magica as a crude weapon, the energy can be directed without care. It will cause harm upon striking as a rule.

However if you want to perform something more sophisticated with it, using it like a scalpel instead of a hammer, then you want as much support as possible. Holding the intention clearly in your mind is important, and further reinforcing it with your words has been demonstrated again and again to help.

Although they are monstrous, and portrayed as such, Demons practice Verbal Magic often. They (unfortunately) have access to spellbooks and teachings that are often far more sophisticated than Humans. Be aware that Demonic Magic, when reinforced by Demonic Speech, is highly potent and not to be underestimated!

-Intermediate Lessons in Magica, from the Grand School of Magica