Although it appeared as a brain there was no doubt more to it as there was a heartbeat and it did have a vocal system (although according to the Rani it only spoke when it had something intelligent to say, unlike the Doctor) and presumably had an audio system to hear orders.
The Rani was using the brain, along with scientists from Earth, to calculate the formula needed to create a lightweight substitute for strange matter, necessary for her plan to convert the planet Lakertya into a time manipulator. The brain is not infallible, and when it came up with an incorrect calculation the Doctor corrected it. It was destroyed when Beyus placed some of the Rani's explosive ankle belts around its base; the Rani, infuriated by the Doctor and not realising, detonated the belts. The explosion also destroyed her laboratory and the Brain.