When border collie puppies play, they learn many of the qualities they will later need for working: discipline, concentration, patience, respect, speed, control, observation, reflection, on and off switches, team work...
I like to see a distinct difference in attitude already in my puppies between when they are just goofing around and when they are in work mode. When they are working I don't want to see any excitement, just focus and cool.
In this video you can see this difference very clearly. The pups start out just running around and enjoying the walk, but then it gets serious...