Presumably they are essential because they sell food and medical stuff that you can't get anywhere else if you need something more specialised than you would get in a supermarket.
Exactly. You’re not going to find crickets, locusts and frozen mice in the Morrisons. Well, you might

I have never seen/smelled someone who fries onions as much as the guy downstairs and I really hope he chocks in them someday.

that sounds horrific. And I've smelt some gross things at work.