auto ref function parameters in a free function

[Martijn Pot via Digitalmars-d-learn]

On Sunday, 3 August 2014 at 19:25:32 UTC, Vlad Levenfeld wrote:
> This would make the function always take its argument by 
> reference. I'm trying to use the feature here:
>
> http://dlang.org/template.html
>
> from the section Function Templates with Auto Ref Parameters

I thought I finally saw a question I could (help) answer...

What is the benefit of 'auto ref' over 'in' as you are changing a 
nor b?