Not an Issue, implementation question

[ikouchiha47]

Hey.
I am quite new to vimscript. I found this while I was implementing my own.
As I was going through the codebase. I found these lines quite intriguing.

My question is why such a complex substitution was required?
Why not take each line, see if it starts with l, and if it does then substitute %s with the line?

Also, why is this !~# \\ needed? And the use of \d?

Thanks