Cabal Overseers act as offensive support units on the field. While strong on their own their expensive cost makes them not a first choice for mass production as main strike force. Therefore Cabal uses Husks instead. Husks (all kinds of them including Vanguards) without Overseers are not very efficient. They only have a basic combat AI assigned and are mostly based of corpses. Overseers have however a superior self aware AI, who are the main battlefront commanders, they modify the targeting and group behaivor of Husks in their surroundings. As a result those Husks get improved rate of fire, range and accurary - makeing them truly a force to be scared of.