One thing is flea prevention. We keep the dogs on it year-around because they're indoors and outdoors. The cats we generally have off of it in the winter months, since it's a) too cold, and b) they're indoors.
So far, this has worked for us. But it's still expensive.
An old vet of ours told us that in certain brands of flea control, the same percentage of active ingredients are in the cat AND dog controls. Which means, instead of buying 3 packs per month of the cat meds, I could just buy one "large dog" pack and divide one month's dose between the cats. Doing this with a needle-less syringe makes getting the 0.8mL per cat dosage a little easier.
It saves a ton of money. And it's safe if you do it properly. Which is why I feel really grumpy that my vet gave me such a hard time today when I asked about it.
End thoughts: They must make a lota money on the sale of flea control.