Dollar Tree and Dollar General are sometimes located in the poor part of a city, but most of their locations are in areas which are too poor to sustain good margin businesses. Rural towns with a single road and only 1-2 gas stations, etc. their core business is to offer smaller and smaller portions to maintain profits while rival stores go under. They are so prolific, they can get giant companies (think drinks, household items, and pharma) to create smaller and smaller portioned SKUs over time.

The businesses were originally just exploiting a gap in the market, but then PE realized that they could just buy out these local monopolies.