It means it has to be on the field, and not, for instance, in your hand. It also matters for Trap Cards that apply the effects of Trap Cards in the Graveyard.
This was introduced with Rise of the True Dragons—over a year ago.
It likely exists to help differentiate between effects that negate the activated effect of a monster even if that monster activates that effect by removing itself from the field, from effects that negate a monster's effects while that monster is face-up on the field.
Set thing: Hm, I suppose that makes sense, since you could not activate it from your hand since it isn't in a column at that point. Niche, but important I suppose.
Negate thing: Which is why I wish people told me about changes, I didn't pay much attention to the changes for long since they rarely made any, though I now check all new sets due to amount of changes they are introducing lately.