I'd imagine the discount is to be a good corporate neighbor coupled with a monetary benefit (suggestions above like bringing family, coming in slower times, additional spending from food and incidentals - although I suspect the per person on that may be lower than other guests).
I don't know that it's all that common to have local discounts at other venues around the country. I'm hardly a scientific sample, but I live in the St Louis area, which is not a tourist destination, and our Six Flags has no local discount. You can get season passes for a value, but anyone could get those, not just locals. Most other venues around town have no local discount, presumably because the majority of visitors ARE local, so there's no benefit to discounting most tickets you sell. We often visit family in Chicago and it's similar there - no locals discounts usually - except sometimes free resident days once a month or so. I'd calculate that those days are again about keeping up a good corporate name, getting locals in the door so they tell friends and family about it and attract more visitors.