There is only a small amount of Vibealia stored in one's body.
Even if you were to use all of that, you wouldn't be able to execute any significant kravte.
Therefore, to get any real results, using the mana in the atmosphere is an absolute must.
Adding to this, the amount of mana you can commit at once is limited, so techniques that create "synergy" are vital in order to bless more complicated kravte.
It's no exaggeration to say that the outcome of a fight can be determined by the combatants' abilities to use synergy.
Further, there is a saying that maximizing efficiency of the use of mana is basic, common courtesy to the gods.
Vibealia is incredibly delicate; the slightest imbalance caused by outside factors or sickness resulting in a sudden change in the amount stored in the body can even cause death.
Additionally, running out of Vibealia will also result in death.
Once used, Vibealia will not recover itself naturally, and there is generally no way to replenish it externally.
This is also another important reason for maKing use of synergy.
Manakravte, is a way to create miracles on this world, in exchange for a portion of your own life.