As an employee in an airline that is currently operating many repatriation flights, I can confirm both answers from @jcklopp and @DJClayworth and add one more reason, that is the health briefing.
Repatriation flights related to the COVID-19 situation have very special health precautions and passengers must be briefed in advance before boarding. This includes masks and gloves distribution, seating policy (one seat must be empty between passengers, etc.) and lavatory usage, food distribution and collection. Also, identifying potentially infected passengers.
Most of these temporary precautions must be adhered to once onboard, otherwise, there is no use of them, so passengers will be briefed by the ground staff in advance, which makes the boarding takes a little extra time than usual.