[sa547]

Originally Posted by mizou

Mami said that Grief Seed has another function than to purify the soul gem !!
what's it ?

Assuming that you've watched 9 episodes (you must watch carefully) so far...

Spoiler for Soul Gem Secret:

...after Contract, apart from making it easier to use magic, QB takes away the soul from the body, and then transfers it into the Soul Gem, thus making the MG's physical body nothing but a shell, a puppet.

Viewing that the human form is fragile, QB claimed that it is a better system that way when fighting, for the Soul Gem keeps the soul safe (supposedly) while the physical body (he calls it as "hardware") can take heavy damage and be regenerated by magic, even if she loses blood or brutally dismembered -- as long as the Soul Gem is not damaged (physically or contaminated completely by dark sin) nor lost (the physical body goes dead if the Soul Gem is kept too far at a distance of more than a hundred meters -- the Gem must be kept close to the physical body at all times).