Before launching satellites or a space mission for instance to research the surface of Mars thorough testing needs to be performed in advance. Goals of these tests are not limited to investigation of device performance in outer space eg vacuum, but also how they will behave at large temperature differentials. The temperature range can vary between just a few Kelvin to a couple of hundreds of Kelvins when exposed to the sun.
In order to perform these kinds of tests on earth, and depending on the size of the space craft sometimes large or huge vacuum chambers are needed. Besides evacuating these large vessels to as low as p < 10-4 Pa, the test unit itself in addition to the equipment required to maintain these temperatures like shrouds need to be evacuated.