Why is simple. Do you really want to roll all the way to Walmart or Target or even the grocery store for one or two things when DG is closer, and faster in and out with a lot of those quickly needed items without paying crazy convenience store prices? Of course you don't.
I need a bag of chips, maybe some melatonin or band-aids, who knows, maybe a can of Campbells soup, some peanut butter, whatever. Its there, I'm in and out in three minutes. Interesting history behind the whole chain. People confuse it with the one filled with nothing but Chinses crap, but its really a whole different game.