Changing a chain manually seems daft when the idea is to keep moving, but as I say, I don't profess to understand mountain biking.
It does remind me of something similar I saw years ago.
A mountain biker offered to guide me around what was probably a fairly low level trail.
We came across what was - to me - a really steep bit, so I stopped.
He stopped, and told me he would demonstrate his new piece of kit.
His bike already had a triple, but he had fitted a tiny fourth ring, which I recall he called either a ghost or shadow ring.
From memory, it was manual shift.
Off he went, spinning as fast as he could, but not really getting anywhere.
It appeared to me the gear was so low it was difficult for even an experienced cyclist to use it and maintain balance.
"Useless" was his verdict.
A lot of new bikes now come with SRAM 1x11 setups. With the closer ratios available and bigger cassettes, you can have the gear extremes more or less the same as a compact XC double. Downhillers have used 1x for years, but then you don't need any climbing gears on a downhill bike.
SRAM have been leading this for a couple of years, with 32, 34 or 36t front and 10-40 cassettes. OK, your biggest gear is a bit lower but not many leisure riders want or need a 35mph top gear. and the advantages of a 300g or so weight loss, from mech, shifter and cables, plus simplicity, plus the added bar space for dropper remotes means it has become a popular option for a lot of riders.
Off road, many bikes needed some sort of chain retention device, as a bike descending over rough ground will suffer chainslap, chainsuck and chainjump unless the chain is held in place by a tensioner and/or a top guide. To mitigate this the manufacturers added a clutch mech into the mix. The mech spring was reinforced when required by a clutch mechanism which prevented the chainslap, keeping the chain taut. To overcome chainjump or loss the front ring was given a "narrow-wide" profile, the teeth having a varying width machined into the profile effectively binding the chain in place. This now means that the bike no longer needs a chain device or tensioner.
Aftermarket suppliers soon cottoned onto the popularity/advantages of 1x. Without all those ugly tensioners and chain devices bikes looked cleaner, were lighter and simpler without front mechs. Riders with quads of steel were soon converting their triple or double setups by removing the front mech, outer and inner rings and adding clutch mechs and narrow wide front rings. It was pretty common to see 10 speed bikes with 30, 32, 34 or 36t n/w chainrings mated with 11-36 cassettes. This setup is great over most terrain, but the climbing gear became an issue. Unless you were a riding god, even a 30x36 gear is a bit hard on a lot of climbs. You either became a stand-up climbing monster, or you got off and pushed up the steepest bits. A 30x11 soon spins out on a level or downhill bridleway or fire road.
The next development for the leisure rider was the extender cassette cog. Available in 40 or 42 teeth this is placed onto the freehub before the cassette is fitted, then you remove the 15t or 17t cog. Great for climbing, 30-42 is more or less the same gear as a 22t granny on a 36 cassette. A bit lumpy with a jump from 13t to 17, or 15-19, this can be smoothed out a bit with an aftermarket 16t cog in place of both the 15 and the 17.
My Ibis currently has a 32t narrow wide front ring, and a 10 speed 11-42 rear cassette and a clutch mech. I get off and push occasionally, but no more so than when I had a 24-36 bottom gear.