The idea is to postpone grinding until you get the 6 leaders you want and all the soldiers to fill out your team.
Also, even though your Battle Rank will rise quickly the fact is you will get more stats from it. For example if you are fighting a relatively easy monster and at the end it says your strength went up, in reality it might have went up 1 point. Whereas if you are fighting really hard monsters and it says your strength went up, it went up 5 points. So even though your battle rank rises quickly, your stats go up faster than if you are fighting weaker monsters.