Steph, as a rule hospital emergency rooms cannot turn people away. So, people who do not have insurance and do not have money, and thus cannot get in to a doctor's office or clinic, go to the emergency room. If a community does not have a free or low cost clinic, they have no where else to turn.
Others, of course, abuse the emergency room, maybe because they don't want to miss work by taking time off to go to the doctor (or because they can't take off), or don't have a regular doctor, or any number of reasons. It's frustrating because most of us know what is appropriate for an ER, or think we do anyway, but it's hard to judge, also.
I agree, there should be some sort of clinic system that people can go to if they don't know where else to turn. My own health insurance has an on-call nurse, 24 hours a day, on the phone, that is a good initial source for deciding what needs to be addressed right away and what can wait. But, of course, that's part of my health plan, not available to the uninsured.