To further elaborate on that:
It doesn't seem too powerful on first glance, but it is not bad at all.
One of the best examples is charges: the unit cannot fail a 3" charge anymore.
Or if you look at the Engine of the Gods: with a Slann near and using that spell you have much more control over the Engine's ability, making it much more reliable.
Combined with a Balewind Vortex and an Astrolith Bearer it makes Kroak's lowest possible casting roll a five (!!) Which essentially means you cannot fail arcane bolt, even without rerolling.
When cast on a Slann it makes it more likely to roll that double for the extended spell range.
When carrying the Incandescent Retrices Item the chance on that bad result is halved.
When a Carnosaur hits with its jaws, buffed by a Serpent Staff, you could increment a dice showing a five to make it a six, to do double damage.
Those are just a few examples that show how powerful adding just one point can be.