At that point in the process there are two reasons for adding K-meta. The main one is as an anti-oxidant to protect your wine. The secondary reason is that K-meta will temporarily stun or slow yeast. The K-sorbate can then shut the yeast down. K-sorbate will not do much to a really active yeast colony (or course neither will K-meta), but both together stands a better chance.
BTW, this is why you can't stop an active ferment just by adding K-meta and K-sorbate. You need to add them both to a finished ferment to prevent fermentation starting again.
You have to add them both when the wine is completely finished and totally clear. Why ??
Sorbate just makes sure that the few remaining living yeast cells in your wine can not reproduce and start a new colony when you add sugar for sweetening.
So by using sorbate you will have the yeast under control.
However sorbate does nothing to keep bacteria under control. And there are bacteria that feast on things in the wine. These can give the dreaded geranium smell in a wine that has only sorbate.
So to keep bacteria under control sulphite is needed.
Sulphite however will do no harm to the yeast.
That is why both are needed when sweetening a wine.