Callstack is very useful when Inspecting the program state. You can view the function calls that are currently on the stack.

The Call Stack window displays the name of each function and the programming language that it is written in (e.g. PHP). The function name may be accompanied by optional information, such as file name or line number. The display of this optional information can be turned on or off.

A yellow arrow identifies the stack frame where the execution pointer is currently located. By default, this is the frame whose information appears in the source, Locals, Watch, and Autos windows.

If you want to change the context to another frame on the stack and inspect the program state in the different frames.

Note: Changing the frame won't influence expression evaluation, which is always evaluated against top stack frame. However inspecting variable values will work correctly with the selected frame.