But realistically, because if they control how you use your machine, they can start skimming profit off of those digital services every time you print something. That's only works if they have control over how you use the machine in your house.
To outward appearances, they seem to be trying to recreate the printer ink/razor blade business model on 3d printers. https://en.wikipedia.org/wiki/Razor-and-blades_model#Printer...
To reiterate the GP's question: why would I want to do that? In practice whenever I want to monitor anything from anywhere, I just VNC or RDP to my PC.
If it's just about ease-of-use, as the other replies suggest, and not actually gating the functionality of the hardware, I'm having trouble understanding the outrage. It sounds as if they are trying to be the Apple of 3D printing, while also still supporting "sideloading." If the hardware itself isn't locked down, why does anyone care what they do with their cloud service?
OTOH, if the hardware is locked down, then that's what people should be complaining about, not an optional cloud service.
Bambu Labs however, has chosen to market their printers with an app that provides a "one-stop shop" for all things 3D-printer. You can browse their version of Thingiverse (or Printables or Cults 3D) and send jobs directly to your printer. You can also access your printer remotely (read outside your home network without tunnelling/port-forwarding/VPNs) to monitor prints, get notified when a print is done, get notified you've run out of filament, watch the printer work if it's equipped with a camera, etc. etc.
Bambu has been attempting to remove features that enable easy local (not-internet-connected) use cases and force everyone to use the cloud, etc. Or at least make it as painful as possible to skip the cloud.
Relevant context: X1C owner who did not update the firmware that forced bambu's "secure printing" workflow on users that previously used their local network "plugin".
I stopped using Handy, blocked the printer's access to the internet, and ultimately, did not miss a thing. The printer continued to work fine with my slicer of choice (softfever's fork of Bambu Lab studio's fork of Prusa Slicer's fork of slic3r, now known as OrcaSlicer).
Like most things these days, they make a decent printer, but are part of tech's steadfast march to control everything. The twist is that they're in a space defined very much by breaking control.
Not defending Bambu. The UX is quite straightforward and easy, however.
So it's a nice to have thing, but it could have very easily been optional. Instead they made it so that every print, even ones sent from Bambu Studio, has to go through their servers(unless you enable Lan mode)