As a campaing benchmark.
Supose on the end of ur first campain, u let 43 terror missions happen and shot down 78 UFOs (just made the number up).
Then on second campaing, you get better on fighting harvesters, get only 27 terror missions and manage to shoot and rescue 93 UFOs.
A nice improvement. So u have some kinda of self campaing benchmark along with the date u manage to finish it, Bases built, money made, etc.
Maybe it could even show on statitics screen.