It's purely for readability.

For me seeing `function()` look the same as `calling()`. It seems your are calling a function instead of defining it.

To make it simple to understand the style guide:
- A function definition should have no space between the name and the left parenthesis. And should have a space after the key word `function`.
- A function call should have no space between the name and the left parenthesis.

It's that simple.