Why You Need to Kill All Fiends?

In Soulkeep you have a mission to kill all fiends and you think it is that but you have a 10 hearts at least with you so in every attempt you may lose some hearts and in turn you would lose some mana and this in the battle but there is something bigger you have to look for it.

You have to look at kill all fiends to get their souls as the more fiends you kill the more fiends soul you get and this would help you as I have read in progress to next level of soulkeep as you have to play more and kill more fiends as even you pass 5 levels of league 1 then you have more fiends souls to get to league 2 although you have completed all levels.

If you do no have enough fiends souls then you have to buy a box or you have to wait until tournament is opening and then you could get more fiends souls while you are playing in the tournaments battles as you have to conditions to pass league 1.

This is what I have read about league 1 and I think league 2 is like it also as you have to play and pass levels and also take care of fiends and this is why I have said that you have to kill all fiends that attacking you.