Futility is the inability of a clinical trial to achieve its objectives.
Problems such as difficulty recruiting enough patients can mean that a trial will not give a result that can be properly statistically analysed. If such problems are discovered during the trial, it may be appropriate to stop the trial early. This kind of calculation is known as a futility assessment, and is one kind of interim analysis.
Stopping a trial early because it is unlikely to achieve a statistically reliable result is ethically appropriate because it prevents exposure of patients unnecessarily to treatments or other interventions. It can also save time and therefore costs.