Bob is part of the Mafia, and some of Bobs friends have made it pretty clear that if Alice testifies against Bob, something unpleasant will happen to Mike, Alice's husband.
The idea is that the 3rd party must testify, to prevent this sort of issue. Wont stop it I am sure, but there are other things that can be done to prevent it (sealing the records so nobody is sure what Alice said for example, vs some of the other witnessess, if early enough even preventing it from being known Alice is a witness.) Any choice you make can be turned to good or evil, in this case its maybe not in the defendants best interest to let the guy testify, but he has no choice. In other cases, it might be the reverse. Pick your poision, at least this way the goverment has a harder time nailing a 3rd party to the wall if they testify on something they dont want coming out, at least from a legal standpoint.