A primary role of a supervisor is to see and to anticipate the hazards and risks of a job before they become a problem and then communicate effectively with their team to prevent these hazards from ever causing a problem.  If a supervisor cannot do this because they don't have the technical expertise, the time in their day to oversee the work, or the leadership capability to manage their team, then this supervisor is unable to effectively do their job.  These are critical tasks for a supervisor; to understand the risks associated with the work, to anticipate how bad things can happen and then to intervene with the team to prevent them. This is the primacy of supervision.

Supervisors must be familiar with the technical aspects of the work being performed, the laws and regulations that govern the work and that are applicable, and the company processes and procedures that are also applicable to the work involved.  And with this knowledge supervisors cannot simply parrot the requirements to the team during the toolbox talk, but actually oversee and supervise the work in the field to ensure that the processes that define how the work is to be done are followed and stray / unaccounted for risks are minimized and then mitigated or managed when they do occur.