The Campaign Last Updated field displays the day, month, year & time that an admin updated the campaign. Anytime someone changes something in a campaign, this date will change to reflect the current date.
This is helpful for several reasons. First, it's helpful to have this information when looking at annual campaigns to see if you have made changes reflected for the season. It is also helpful to see if your campaign was updated on a date that was not performed by you and you can inquire with your colleagues about the changes made. Lastly, should there be an issue, it is helpful for the support team to know when the last changes were made to read logs for troubleshooting.