As far as I can tell:
It's vaguely worth it if you have multiple devices on a monthly AppleCare+ plan, at which point today you're being overcharged if you leave it be. This allows you to consolidate devices into a package deal.
Also, if you have devices released within the last 4 years that missed out on the AppleCare+ deadline and are still in good shape, you can put them on the AppleCare One plan as one of your initial three devices, or an additional one.
It's not worth it if you've already prepaid a year or so's worth of AppleCare+ coverage on a device, which they will refund you at a prorated amount and ultimately lump you into a subscription plan where you'll end up paying more.