There are a number of reasons you might prefer an architecture like this. The main two are resilience (don't place all your eggs in one basket -- if your computer is damaged or lost you don't lose your data) and availability (if your computer is shut down you can still access your data from your phone, tablet, or another device).
It would make great sense to have a non-hosted backend option, though, for power users who prefer to self-host their data.
It would make great sense to have a non-hosted backend option, though, for power users who prefer to self-host their data.