Yeah, you just use them to remove any elemental blights you might get. So for instance, if you get hit by a fireball, you're health would drain, you can take a berry or roll to extinguish yourself. Ice and water I think makes your stamina drain faster. Thunder blight makes it easy for you to become dizzy. Dragon blight reduces your attack power and makes you have really bad negative affinity (at least, that's how it was in Tri). So yeah, nulberries are the one stop cure all for all of those ailments. In Tri you had to have a berry that was matched for each effect which was cumbersome.
Also, as stated earlier, these are also a good money maker, because you get good zenni for minimal resource points.
Just a few corrections to what you said:
Water makes your stamina recover slower. Ice makes your stamina drain faster.
Nulberries do not cure Dragonblight. They still have Dragonfell berries.
So GAF I need to know, how and when am I supposed to be able to forge/buy a Kelbi Bow. Is it just possible on High Rank or what? Haven't seen it so far.
From what I understand, it is end-game stuff. Not entirely sure.