Occupants introduce realistic loads to a house system. Occupants introduce sensible and latent load through their body heat. In-house activities, such as turning lights on and off and using home appliances and miscellaneous electrical equipment, also introduce sensible and latent load. Opening and closing of windows brings in natural ventilation.
Occupant behaviors, because they're varied and difficult to predict, also create the biggest uncertainty and challenge in testing, monitoring, and analyzing building system performance. You may want to simulate occupancy in a test house to test the performance under controlled conditions. This is also useful when you need to test a new system that may not provide adequate comfort conditions.