Assigning tasks to devices can be done due to the capabilities of each device but also due to other factors, like what behaviour you want to influence. For example, if you want to spend less time doom-scrolling/on social media/whatever, moving these tasks outside of the computer you have in your pocket and into the computer you need to sit in front of helps.
In fact, it kind of runs the other way: even my "portable" "real" computer is terrible as, say, a camera, or level. It's a bad GPS navigation device, both due to the form factor and it's entirely lacking the hardware for it (technically they can have this, but very few do).
There are lots of things my phone can do that even my laptop, let alone my desktop, practically can't.