The great majority of callbacks in MATLAB are called with two automatically supplied arguments.
The first argument is the handle to the object which the callback is about; this allows you to use the same callback function for multiple objects and still reference the appropriate object.
The second argument is either empty (most common) or a struct . The content of the struct will provide details about what triggered the callback. For example KeyPressFcn callbacks get an event structure that indicates which key was pressed.
It happens that the timer object callbacks never need to be passed additional information of this sort, so for them the second parameter is always empty, [] . MATLAB provides the argument even if it is empty in order to provide a uniform interface. It is not required that you do anything with either parameter.
(The exception to this organization are the few "callbacks" that are used as filters, such as position constraint functions and datacursormode datatip construction functions; these few functions return values whereas regular callbacks that follow the standard scheme never return values.)
