You can give parameters default values:
def greet(name, greeting="Hello"):
print(greeting + ", " + name + "!")
greet("Alice") # Hello, Alice!
greet("Bob", "Hi") # Hi, Bob!
If you don't provide an argument for greeting, it uses the default "Hello". If you do provide one, it uses yours.
This makes functions flexible. Common cases use defaults; special cases override them.