Why specify parameters' default values?

Say you have a function

int f(int a, int b=3)

Then pybindgen allows you to write

module.add_function('f', retval('int'), [Parameter.new('int', 'a'), Parameter.new('int', 'b', default_value='3')])

Why would you want to specify what the default value of parameter 'b' is? Why not simply have a boolean flag that says "this parameter has a default value"? Is it because this saves wrapper code, as it doesn't have to check with how many arguments a function is called from Python in order to pick the correct invocation?