They help force you to have better kicking technique and build muscle, just like how paddle do for your stroke. I use fins plenty when I'm working out. Zoomers are especially helpful since you are even able to do breastroke in them, which is obviously impossible with normal fins. Don't use them all the time, but they're great for portions of a workout like doing one-legged spin drills on your bike or doing strides for running.

I think they're great to use for drills or specific parts of your workout. Just don't come to rely on them. I see people who use them almost continuously, as a substitute for not having good body position or technique in the pool. Personally, I try not to use any training aid (flippers, pull buoy, etc.) for more than 25% of my yardage.

I think zoomers can be useful for building leg strength and also for giving you speed during drills. But I also think that kicking with them is not the same natural kick that you use without them, so it's good not to use them too much.