There are times that I go a long time between uses of my conical disk.
But in a nutshell it does do an amazing job of "jointing" stuff with crazy grain. Maybe a jointer with a helical head would do as good a job without tear-out but I would not know since I don't own a helical jointer. Even if I did own a jointer with a helical head, there are some wood products (plywood/melamine) that I use that I would not want to run through a jointer for fear of dulling/ruining jointer blades. I think sandpaper, even for the conical disk, is a lot less expensive than jointer blades.
Beyond jointing, the conical disk does a superior job of sanding to width than the flat disk because, unlike the flat disk, you don't have to screw up your fence alignment to do so (I gather the 500 fence has an offset feature that doesn't screw up your alignment but the 510 & 520 fences do not as far as I know) and it leaves the edge sanded parallel to the grain, without the characteristic cross-grain swirls left behind by the flat disk.
On the down side in addition to cost of the conical disk and the consumable sandpaper, set up can be a little finicky because you need to get the table tilt to an exact 90 degrees relative to the slope of the conical disc. Even with a good square, that can be frustrating as tightening the table tilt lock can cause a small amount of creep. A Wixey or other digital angle gauge can help with the process. Attach it to the slope of the conical disk, zero it, and then put it on the table and adjust the tilt until you get that perfect 90 degree alignment.
Is it an absolutely necessary accessory? Nope. And it's hard to appreciate its capabilities if you haven't tried one. But it's a good tool to have in the bag along with all the other tools.
Gale's Law: The bigger the woodworking project, the less the mistakes show in any photo taken far enough away to show the entire project!