Guards were used by the Oracle and the Seers on the planet that formed around the P7E to maintain order over the "Trogs". They were primarily responsible for ensuring that the "Trogs" were achieving their mining quotas.
However, they also had general police responsibilities, and could bring charges against individuals not strictly related to mining. Once, they charged Idmon with incitement to rebellion or treason. They also captured and held Herrick on unspecified charges.
Guards were generally gruff, harsh and demanding. At the time that their "planet" was visited by the Fourth Doctor, Leela and K9, they wore full hoods over their faces, which closely resembled the typical hood of a some kinds of European executioners on the planet Earth. Like all residents of the planet, they were descended from Minyans.
Most of the guards were killed when the planet was destroyed by the Oracle's bombs.