Increasingly, calculations are based on timed events originating at many sources. By comparing, contrasting, joining, and noodling over these inputs, you can derive some interesting results. If the inputs to these calculations come from disparate computers (or sensors), you can't always be sure of how quickly the information will propagate. Hence, you can't be sure when you will get an answer to the requested calculation.
If you can't promise when you will get the answer, what are you going to do? You can wait to get the perfect answer, or you can give an imperfect answer more promptly by basing it on partial knowledge.