I've seen this on some skills, I think maybe it's meant to boost ailment if you add ailment damage to it, via added damage mods or something else, maybe. Like for example you have the passive that makes so you can apply 2 different ailments per hit, then you add say toxic damage to the skill via items(toxic to spells on staff), now you can apply curse+poison or weakness+poison for example, in which case the ailment mod affects the poison part. It'd be worth it since ailment damage doesn't scale with the hit, it's a flat value per stack, so even if you only add 1-2 toxic damage to a spell, assuming it's the 2nd highest source of damage, you're now getting full damage poisons.
It's possible they just added some blanket mods and didn't double check this kind of stuff though, there's a bunch of broken or suspect mods.