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.
(from wordy) Steve