When I'm just getting from Point A to Point B, finding a campground for the night is a hassle, as well as an expense. Compared to a typical campground, Walmarts are :
Easier to find and access
Always level, pull-through parking
Friendlier staff
Always answer their phone
Better camp store
Better security personnel
Cleaner bathrooms (though not as well-equipped)
No check-in, nosy neighbors, kids running around, smoky campfires, or loud singing into the night
I'll typically head out early in the AM, do touristy stuff mid-day, then finish the day's mileage quota later, on my way to the next WM. Living in the northeast corner of the country, I often have to travel a couple of days just to get the "real trip" started!