That and the increase in viscosity.
The increase in stability is significant n another way: it allows you to hold the sauce well, and it even allows you to make the sauce in a quantity, use what you want, then cool and fridge the rest for your next use. (You could, e.g., make a sauce base, remove some and flavor it they way you wish for today, cool and fridge the rest. Next use, remove some, flavor it differently, and so on.
Sauce beurre blanc is a pan sauce, made à la minute, meaning it's made at the last moment, when needed. Only a very small quantity of liquid is used (the original reduction of the liquid to practically nothing in the pan) because more would mean substantially more butter would be necessary - and significantly more time, if it would come together at all. A beurre blanc can simply not come together if the liquid is too much, and/or it can break if whisking is too vigorous, if overheated, if held too long.
Xanthan changes the dynamics across the board: it allows the use of a blender, meaning one can simply purée in the original seasonings/aromatics; it allows the use of more liquid; it allows stability, meaning it can be prepped in advance, held, stored, etc., and should it break, a quick plunge with your handblender will fix it; it makes the sauce adjustable on the fly, meaning one can increase or decrease viscosity at nearly any time.