Combined chip with LDO and Buck, what use?
Looking at the application circuit, it seems that they have made on channel an LDO and another a Buck to save cost and component count.




Core need higher current and lower voltage than I/O.  Buck works better for that than an LDO.
I/O needs somewhat higher voltage and less current (usually).  LDO would be adequate.


Why not 2x bucks?  Because a buck requires more components (inductor and better capacitor) than LDO.  In this case, the combination of buck and LDO is a compromise of efficiency vs. cost and size.

It's not uncommon to use an LDO to power RF communication, analog front ends, because these can be susceptible to the EMI.  At the same time, the digital section can be powered from a buck.