One possible reason is that it facilitates trust-building. Say you are stuck in a cell with another prisoner, and every day you have the chance to cooperate or defect on a small task (for example, sharing food equally vs trying to steal an unequal share). Later, you are asked to testify against each other and get a slightly reduced sentence in exchange for the other person having a drastically increased sentence. A history of the other person cooperating gives some evidence that they will cooperate in this new situation as well.

Another analogy to this would be the process of building credit. If you take out lots of loans and pay them back scrupulously, you build a history of credit worthiness. The banks are more willing to be vulnerable based on past behavior of not defaulting.