Can somebody explain to me why I would buy these two spells? because you can also just kill your conjured creature instead of buying the spell book. Is it because some conjured creatures are too strong and will take some time to die?
Those spells, as far as I am aware, will work on hostile summoned creatures, as well. So when you interrupt a cabal of evil necromancers and they all summon creatures, you can blow away the creatures with one spell each and then decimate the enemy magi while they're low on Magicka.