It's 1975 in America. You're on a family road trip. It's hot, you've been driving all day, and you've got two whiny kids in the back seat. Everyone is getting hangry, and you need somewhere comfortable to pause. You want somewhere where you can sit, get something that the kids will eat without fuss, and maybe an ice cream would be nice. The obvious answer is Howard Johnson.
Drive throughs didn't exist at this time, and even if they did, you wanted to get out of the damn car anyway.