Swarms are basically how honeybees procreate. Strong colonies usually swarm in the spring leaving some bees at the old location and queen cells to hatch. If all goes well, the queen cells will hatch, the virgin queen will mate and take over the old colony. While the old queen and the rest of the bees swarm to a new location to start a new colony. Swarms are considered great comb builders because the swarming bees will gorge themselves on honey before they leave the old colony in anticipation of having to start from scratch. So when they find a suitable location, their wax glands are in full swing. The reason they are such prolific wax builders is that it takes almost a month from when the queen lays eggs before new bees start to hatch. So during that time, the colony is in constant decline as older bees die off. Considering that the average life of a bee is 4 to 6 weeks during a flow, you can see why it is important for comb to be built quickly so that the queen can start laying. This is also the reason many people prefer buy nucs instead of package bees. With a nuc comes brood, so the colony will grow from the first day and not be in decline for a month.