Your Mac can do what iPhone and iPad cannot. Multiple users. Ron explains what iOS does not allow:
Multiple user capability. On macOS, you can add multiple users; dozens if you choose. That means the Mac can be a family machine that provides privacy and security for each user– Photos, iTunes, Mail, Safari, Calendar, Reminders, Messages, FaceTime– all segregated by user, each with their own preferences and settings.
Why doesn’t Apple allow multiple users on iOS?
Apple is a hardware company. As an example of where a multiple user login would work well is the iPad. Many households have multiple iPads. Why? Each user wants their own iPad. If multiple user capability was available, a single iPad would work well for each user in a way similar to the Mac. Apple would lose hardware sales because customers would not need to buy as many iPads per family.
So, it’s all about money, not the customer.