I suppose there might be special cases when you need a small device and functions dumbphone simply can't offer (say, internet messaging or voice calls), so it's offered as a "backup" to the main device.

Personally I'm not a fan of small devices, but if they manage to ship, there is a thing I like about that campaign. They basically took the cheapest MediaTek chipset available with 4G and managed to keep the price competive with budget smart phones on same SoC (at least for early Kickstart bakers) instead of overpricing it for "smallness". This pretty much fits its usage paradigm as second/backup phone.

Actually there are MediaTek-based smartwatches running Android, which are technically phones, but usually they run on way older hardware (like MT6580 with 512MB RAM).