Motivation!

Their enemies were only a bunch of X-Mortal Rank Beasts, Two-Tailed Rats. However, Ban won't underestimate them due to their low Ranks as Two-Tailed Rats relied more on their strength in numbers than their individual strength. 

He thought that using two Reinforce on Eva was the right thing to do as on one note, what Eva feels, he would also feel it due to their Perfect Harmony connection. Second, even though Eva was stronger than them while still on the same rank, he knew that sooner or later, one or two attacks from the Two-Tailed Rats would get past her defense. And third, the Skill 'Reinforce' has a cooldown of thirty minutes, and its cooldown will only start counting down the moment he already used all of the stacks. 

He used it instantly so that he could save time and let it go into cooldown immediately. However with the double Reinforce, one could say that Eva was actually invincible right now as their enemies only consisted of low rank rats.