Configuring the max\min server memory is for the whole instance, while resource governor is used to limit user connections. You may want to limit the max server memory when you have more than one instance or in case that other applications are running on the same machine.

Resource governor is useful when you want to limit the resource used by certain user connections g.e. a reporting system.

Also remember the percentages you assign through resource govenor are a percentage of the max memory, not the physical memory in the machine.