Feature for importing Leads or Contacts?

Hi all,

Just stumbled across a feature I wasn't previously aware of.  When viewing either a Lead or a Contact record, clicking on the top-left icon (Le or Co) seems to initiate a file import function.  I can't figure out what exactly this function is trying to achieve.  I've tried several file formats (doc, csv, vcf, msg) and got nowhere.  The error message is useless and I can't find any help to explain the function.

Can anyone explain what this function is for and how to use it?

Thanks in advance,

Dave Hodgman