As Kiya says, it's a way to raise your resistances.
Basically it's just a bit of role-play fun as the bag doesn't hold very much. In practice, the resistances you gain will make almost no noticeable difference to how the fights pan out as the numbers are too small. I could never see any difference if I dropped the bag and tried a fight without it.
I guess if you were really obsessive you could try and collect enough crystals to fill your bag with one type, and swap them around to suit whatever monster you were fighting.... But it's not necessary. More a bit of extra fun than something that requires you worry too much about whether you've got the right crystals, right combinations, etc. <img src="/ubbthreads/images/graemlins/smile.gif" alt="" />
