As to the questions of why one would want a convertible helmet:
Like DC1988 I like that I have a half shell, a full face and on rare rides, the option to convert mid ride.
It saves me the money of buying 2 helmet. Given the price of high end helmets, that’s not nothing, especially for the more casual rider. If you are racing top level events, you can dial in your kit just for that.
But for me? Couple of bike park days a year, few club enduro races, and one club DH race.
It also saves me storage space, and especially, travel lugage space.
Going on a 4 week trip, with 3 days of XC riding and a day or two at a bike park? Sure is nice not to have to pack a bulky full face AND half shell helmet in my lugage.
There is one local trail system where I regularly use the detachable chinbar:
The trails from the trailhead out are regular, rolling XC: up and done, not to scary.
But, in the middle is a rock outcropping with some serious black and double black Diamond trails descending off it.
It’s nice to start in open face mode, and once I get to the top of the rock, ride some of the gnarly trails in full face mode. Then, for the remaining loop of XC trail, it’s back in half shell mode.
Don’t take this as that I am against the fixed chinbar, enduro style full face helmets, those are great too, it’s nice to be able choose for yourself.