This has two main functions:
As a "dummy load", it takes power from upstream and turns it into heat via a bunch of lightbulbs. Useful for generator/inverter load-testing, circuit heating, etc. In this mode, the outlet on the front is bypassed. (Turn its switch "on".) Enabling more lightbulbs raises the load.
As a "ballast resistor", it's placed in-line between utility power and a device-under-test, and the lightbulbs act to limit the current. The DUT is plugged into the outlet in the front, and the outlet's switch is turned "off". Enabling more lightbulbs lowers the series resistance and raises the power available to the DUT.
Open the lid during use!
The lid is just there to protect the bulbs from damage and dust while stored. It absolutely needs airflow while in use.
Grab a photo!
Add a fan and intake/exhaust grilles?