Another common reason to use disclaimers is simply to correct or change a will by realigning what the deceased would have wanted with what the legal documents ended up containing. Maybe circumstances changed, someone got divorced or remarried, or somebody had or adopted a child. Often, we don't think about estate planning when we should (because it's a drag), so those kinds of errors (even typos) sometimes get through.
But by making a few moves with disclaimers, you can set things back on the path your relative would have preferred. For example, even if the husband's will creates a trust for his wife and kids, naming the wife as the executor means she won't get any tax breaks when she passes her estate on to their kids, because it also includes his estate now. By disclaiming her power of appointment, as the person in charge of this trust, she puts things back the way she and her spouse agreed they should be.