Resistance stackers like the Monk, Gorgon, Paladin and Berserker (and anyone if you know how to do it from 0), profit from any amount of poison. They usually stuff their inventories with various junk so anything synergistic that's a small item is welcome. Having good CP also helps.
What it does is increase the value of TT's poison, so you need only 3 hits to get to 5 poison strike (and a dodge + 5 poison strike makes Monk quite a monster), helps with mana regen while you fight (that's a free getindare activation even if you've used 2 fireballs with 12 mana) and helps with getting TT piety (making it easier to get more poison).
And also sorcerers, obviously, if it gives you 3 tiles of regen, that's 18 mana with no powerups, which means 3 fireballs (which at lower levels means another hit), and if you did pick up powerups, that's 18 + 3 mana which adds a potential getinthere hit to the 3 fireballs (and the hit you get from healing up).
And then there's assassins, who want their stuff poisoned as much as possible, and it simply triggers their dmg bonus without having to expend mana.
It's a cool little piece of junk.
Also, there's classes in the game like the priest and the berserker for whom being able to regen a tile or two makes a world of difference as it means they can get one more hit in. And also a potion chugging thief who can also turn a few tiles of regen into a meaningful action because he can pad it out with off-color potion regen. Etc, etc...