There are many and diverse reasons that adult patients seek emergent, life-saving services in an emergency room.  And there are symptoms that could be safely managed both by a primary care provider and 'physician extenders' at an urgent care center.  For adults, here are a few common symptoms and suggestions for choosing where to seek treatment for those symptoms.
Symptoms
PCP
Urgent Care
ER
911
Low back pain
null
 
     
Minor aches
     
Mild asthma attacks
null
null
   
Sprains or broken bones
 
null
   
Cuts, scrapes, wounds or burns
null
null
   
Nausea, heartburn, upset stomach
null
null
   
Vomiting, diarrhea or dehydration
null
null
   
Frequent or painful urination
null
null
   
Allergic reaction or allergies
null
null
   
Rash
null
null
   
Head injuries    
null
 
Adults–Fever under 104
null
null
   
Adults–Fever over 104
   
null
 
Uncontrollable bleeding
   
null
 
Sneezing, runny nose, sore throat
null
null
   
Ear or sinus pain
null
null
   
Chest pain, pressure or pounding heart      
null
Numbness in face, arm, leg      
null
Sudden confusion, trouble speaking or understanding      
null
Sudden dizziness, loss of consciousness or balance      
null
Sudden severe headache      
null
Difficulty breathing      
null
Overdose or poisoning      
null
Vomiting blood      
null
Management of diabetes, high blood pressure and other chronic conditions
null
     

Many times, urgent care centers can also provide immunizations, sports and school physicals, and may even offer radiology and laboratory services.

And before visiting any new urgent care clinic, make sure you know if your health insurance coverage extends to them or if you will be paying out of pocket with no reimbursement.  Many plans incent urgent care visits as opposed to ER visits because, when appropriate, they're typically less expensive. As Benjamin Franklin said, "Penny wise, pound foolish."