A fair amount of users most likely have more than a couple versions of AutoCAD and/or Inventor installed on their system. This may be due to testing the latest version, while production requires the previous or having other business system requirements that force them to retain an older version.
My reasons are simple - providing all inclusive support to our customers by being able to troubleshoot multiple versions simultaneously. But regardless of why the fact remains that by default until now Autodesk Vault used MS Windows settings for determining which [version of a] program to open for a given file type (except 2015R2 for subscription customers which includes some early 2016 functionality).
Autodesk Vault 2016 now gives you greater flexibility in choosing which version of your Autodesk software to open.