Yep, the gap between the point where most people would start to think a skin-infection-caused rash is bad enough that they might need help, and the point at which it's moved deeper in the body and begun to infect other tissue, can be pretty damn short, and by the time you've noticed (say) joint aches or immobility, it may be too late for simple solutions. If you've hit that point on a Saturday morning but think "well it's the weekend, I'll try to see my doctor on Monday" but then it's looking really bad by Saturday night (not implausible) and you can't get to an urgent care for whatever reason (none nearby, or just not open), you really don't want to wait until Monday to have a professional look at it. ER's not a crazy choice. If it's truly bad that's where the urgent care will probably send you, anyway.