Great question, there is a couple different ways to think about this. Are your physical PC's all HAADJ or AADJ? The value of AADJ is that it decouples and reduces the need to maintain on-prem infrastructure, so simplification is a huge reason. The other is PC's don't need line of sight to the AD domain controller, this is a huge advantage for physical PC's that are off your work network so alignment on domain join is a reason along with agility to roam anywhere as long as device has internet connection. Hope that provides some reasons, it really boils down to flexibility and simplification.