I've done a google search on both of them, can't find much information.
I was wondering what is a logical subsystem and physical subsystem (in an object-oriented system)?
Also why would it be useful to keep the two separated?
The logical subsystem is how you would conceptually divide your subsystems. The concepts are important but physical locations don't matter. So, logically the Model-View-Controller is divided into the 3 subsystems.
The physical subsystem is how your subsystems are deployed on the hardware. For instance, the Model and Controller can be located on a server PC while the View is located on a laptop that talks via Ethernet to the Model and Controller.
I would usually assign development responsibilities based on logical subsystems. Also, logical subsystems can span across multiple physical subsystems.