People have processes that put files in standard places and require users to email them the files. Yes, the average HN reader could automate away any need for the average process, but it exists and can't be changed. Email client developers go ahead and implement niceties without considering the risk.
I think the intent was probably for the parameter to be used by desktop applications so that they could generate or find a file and then open an email with the file already attached.
How was this standardized? Why did clients implement this?