why
I'm not sure why the ammo companies even made .410 slugs over the years. They may persist now due to all the .410 handguns (another curiosity in my book), but as a sporting load, I never saw the purpose.
Seems like a .410 slug weighs 87 grains (that is from memory). Somebody will tell us the correct weight and the velocity as well. And neither are much, and launched from a bead sighted, field gun, usually pointed by a kid. A sure combo for failure and disappointment.
I've never seen an adult carry one for deer. I suspect some of that ammo gets bought by folks who only own one rifle, too much gun for a child, and a 12 ga, again too much gun, or maybe no guns at all. But the kid had a .410 given to him by somebody as a gift, so they buy them a box of slugs so he can hunt with Uncle Charlie. Nobody will spring to buy the kid a usable rifle, but they will shell out a couple of bucks for a 5-pack of poodle slugs.