Quite often when an ad in Eros, CV or other ad mall expired, some members report that the weblink is no more valid.  The profile's link will then be placed under the bad websites list with "Report New Website" in the weblink field.  Even if the lady renewed her ad a couple of days later, it would be hard for someone to find that ad again.  If the field of "last known website" exists, members that came upon that profile can then check again to see whether it became active again.

The other benefit for having such a field is for members to track ads by B&Ss and ROBs or provide leads for ISOs.