So obviously it's not necessary at all, but Bambu built their entire brand on ease of use - the app allows you to pick from thousands and thousands of premade models and send them to your printer directly from the app. Judging by the Facebook Bambu groups, most people never bother with installing PC Bambu Studio. And because phones don't necessarily have the raw power to slice the model on device, it's sent to their servers for slicing to fit your printer and filament type.

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)