To handle the tasks that are still too much for a tablet or phone. In my case that is doing video editing and for that even my new top end MacBook Pro is sometimes a bit marginal. I need a Mac Pro but don’t really wanna drop $10k on one. For practical purposes everything else it’s just more convenient to use an iPad or phone. For most people I expect they will use cloud services for any kind of heavy computing tasks. For me I expect that there will be some kind of convergence where iPad Pro or similar becomes the monitor that will talk to some sort of external processor unit for certain tasks via 5G or 6G network.