The vacuum chamber might seem like a complicated place, but that is not what it really is. In this chamber the machine will suck out all the metal into the mold. Note that there is a need for enough force to resist the surface tension of your molten metal.
Unless that is done, your molten metal will just stay there because it will turn into a blob. In the vacuum chamber, there is adequate steady pressure to help you achieve the bottom line without breaking a sweat.
The molten metal is also placed in the vacuum chamber to make sure that there is no air bubbles on the inside. If the air bubbles are not gotten rid of, you will not end up with a product of the best quality.