As Tom says. It is one of the main reasons for doing an A/S.
Swarms = most of your foragers saying goodbye, AND taking a few days honey supply for the swarm. The bees left behind need to delve into the stores to feed the brood left behind.
It also means about 3 week gap in the brood cycle, and perhaps more, so your foragers from the new queen will not be foraging for about 6 weeks from swarming - they will be just about right for provisioning the colony for the on-coming winter!
A/S means that you still have all your bees. The foragers are redirected to the 'swarmed' part for a week, perhaps more, so should be continuing to store honey (at an increased rate until brood needs feeding!).
By then the new queen will hopefully hatching your colony be getting closer to new brooding. After a couple of brood cycles you may wish to re-queen the 'swarmed' section with that queen by removal of the old queen and uniting the two parts. By that time you will have had two queens laying for the past few weeks and so the new colony, headed by a young queen, will be strong enough to collect a lot of late nectar, if available, especially as the brood needing feeding will diminish to half after a week.
I've undoubtedly missed a few of the stages, left the dates open, but you can see the advantages of keeping all your bees. In the unlikely event of that new queen having a mishap - killed, damaged, not properly mated, your colony security may be further damaged, or doomed without serious, lengthy, or costly intervention.
Regards, RAB
