How does "Purpose of sending email" work?

How does "Purpose of sending email" work?

Since today it seems to be mandatory to set the "Purpose of sending email".

When choosing multipurpose - our existing mailing lists cannot be choosen

  • is this a bug?
  • do we have to make new list?
  • how does this feature work?
  • whats the effects of this settings?
It would be quite nice if you provide a tutorial when releasing such important features!