The algorithm may take a while to deliver a result. While this is happening, the user wants to know what is taking place.
The system provides granular progress messages during calculations.
This is important both for the transparency of the system—to reassure the user that the system is operating and hasn't stalled—and to create the impression of an intelligent system with many moving parts. In some cases where there is no actual processing delay, it may even be beneficial to simulate one (as with Demonstrating Thinking). By contrast, sometimes it may undermine the overall impression of the system to expose its workings and over-explain (in which case Appropriate Magic might be deployed).