Consider a 10 story building, and you need to design a steel column, now consider one column line. if you select 10 columns(members) in line and convert them to one physical member and design; the program will design using the maximum values on the most critical section and then assign it to all the member. So for example if the bottom most column (naturally) is the most critical and gets assigned a w14x132, the entire column line will be assigned w14x134.Contrary to this if you use member design each segment will be treated individually.
Secondly after you design the physical member then right clicking and "view calculations" will bring you very nice and elaborate calculations you can view and compare with some basic manual calcs.
Other than that not much difference !