I have booked the 24-hour plan on Villa Fontaine Grand on my next trip to Japan soon.
I will be arriving at HND in the early morning, and I would very much like to have a power-nap around noon to fix my jetlag. (I find the standard check-in time at 15:00 to be a little too late.)
Booked through their website, and strangely enough the confirmation stated the standard check-in/check-out times, but I got contacted by email a short while after to specify my desired check-in time.
Also got the "Spa ticket" option, not sure if this is single-use or not, that remains to be seen.