So I asked Google

[Pudsey Bear]

I was pissed off that this cold is hanging on.

"what is the purpose of the common cold?"

The common cold is a mild upper respiratory tract infection that's primarily caused by the body's immune response to a virus. The purpose of the common cold is to help your body develop antibodies to fight the virus and prepare for future exposure:

• Immune response
  When a cold virus attaches to the lining of your nose or throat, your immune system sends out white blood cells to attack the virus. Your body also produces inflammatory mediators that cause the symptoms of a cold.
  
• Antibody production
  Your body makes antibodies against the virus that caused your cold. If you're exposed to the same virus again, your body will remember how to fight it using these antibodies.
  

The common cold is usually harmless and clears up within a week, but it can take longer for symptoms to go away completely. The main goal of treatment is to ease symptoms while your immune system fights the infection. No medications can get rid of the common cold or shorten the length of the illness.