The supply side is even more straightforward. There exists no supply chain that can handle the diversion of all restaurant foodstuffs into grocery supply chains. The food would literally rot in large quantities, and stores would simultaneously be unable to meet increased demand from all the people who would have been eating fast food.
You provide an easy and cheap meal, which, sadly, to a lot of people is the best they can afford to eat or feed their families with, due to lack of money, skills, time or their living conditions. That’s the reason you are essential.