They serve, almost exclusively, the poor. They end up being the line of last resort for the poor and very poor to eat food, buy toiletries, and other must-haves. They supply a facsimile of nutrition but because they are "cheap" in a way that everyone can conceive of, and because they have a strong supply chain that is affordable for the companies themselves to run, they end up filling in the gaps in much of suburban and rural America, where grocery stores are nearly nonexistent.

Some real interesting stories on their negative effects on communities here:

https://www.wbur.org/onpoint/2019/08/23/dollar-store-dominan...

https://www.forbes.com/sites/pamdanziger/2018/12/30/will-dol...

https://www.fastcompany.com/90278384/why-dollar-stores-are-b...

https://www.businessinsider.com/dollar-stores-taking-over-am...