The Cortical node is a vital implant to Borg. It connects their mind with the collective. (VOY: "Unimatrix Zero, Part II")
It is responsible for operating all the other implants, if it malfunctions they can fail and be rejected by the body. The technology in it is too advanced to attempt to repair, the Borg simply replace it if it malfunctions. (VOY: "Imperfection")