Initially giving the capability for people to author posts in PostyBirb and schedule them for posting at a later date (offloaded into the cloud).
PostyBirb is deep in the v4 rebuild, and is already a massive project. There is not much dev resource spare there as it is, and not much appetite to push on with a web based version - partly around the aspect of being concerned around security and cost, but also the need to have a lot of ongoing maintenance for it. Doing this as a separate project allows it to be ran in tandem, giving the best of both worlds.
As development has continued on PostyFox, it has evolved into a complete solution that is possible to run independently of PostyBirb.