I guess it'd depend on what the agent was, but either way, it means some amount of the salt you're pouring over your food isn't salt. Eliminate the non-salt from your salt and you never have to worry about what might be wrong with it or how much non-salt you're paying for. That's the reason I've switched to shredding my own parmesan cheese instead of just shaking it out of a can since the cans in some cases contain ~10% cellulose.
Table salt is just sea salt (the stuff mined from underground just comes from an ocean that dried up a few million years ago) and is full of other compounds beyond NaCl.
EDIT: I looked it up. It's supposed to stabilize the iodide. The reason they use corn sugar is that it's cheap. Thankfully there are brands that don't include additives in their salt. I'll have to keep an eye out for them. I already get plenty of iodide in my diet.
