Has to do with closure. You finish a piece of business. There's a sense of relief. You can get closure and reduce tension, but the tension is never eliminated because we keep our systems open to be able to explore new stuff. Closure reduces tension.
Some tension is good for learning.
Beyond that, it gets in our way. Another of Lewin's related concepts, made famous by David McClelland in his work on achievement motivation. Using your skills at the level at which they are, you can succeed. Each group, as it starts, has some beginning rules. But each group is an organism.