What is the benefit of using assigning a default value to a parameter in a function in python

Question

Python allows us to set a value to a parameter by default. The following example is taken from A smarter way to learn python by Mark Myers.

def calc_tax(sales_total,tax_rate=0.04):
    print(sales_total * tax_rate)

I don't understand what is its use when we can do this:

def calc_tax(sales_total):
    print(sales_total*0.04)

Answer 1

It's because when doing:

def calc_tax(sales_total,tax_rate=0.04):
    print(sales_total * tax_rate)

You can do:

calc_tax(100)

Then here tax_rate is an argument that's assigned to a default value so can change it by:

calc_tax(any thing here,any thing here)

OR:

calc_tax(any thing here,tax_rate=any thing here)

So in the other-hand, this code:

def calc_tax(sales_total):
    print(sales_total*0.04)

Is is only able to do:

calc_tax(any thing here)

Because there is no second argument