The person near the gates is mandatory (a legal requirement), if the staff aren't available to provide that person the gates will be switched off (so you can just walk through) or they close the route entirely (and if it's the only route that also means closing the station).

The gates are designed so that an adult can force their way through despite the gate being closed, but nevertheless in a fire or other emergency situation you don't want to wait for fleeing people to force the gates, so there must be a trained human available to command the gates to all open immediately. In a tube station there will be a control booth with CCTV of all areas somewhere to direct operations - the gate still must be supervised by someone physically nearby, not solely from the booth. Flow controls that don't prevent you fleeing from an emergency (e.g. the "Do Not Enter!" flashing signs at some tube entrances that can be activated to prevent overcrowding, or the changeable internal direction markers that let them send unfamiliar users over a longer alternate route to their destination) are controlled from the booth though.