COA - What is this for?

Sorry if it's a silly question, but what exactly is a Certificate of Approval for? I realize it's permission to change status once INSIDE the UK...but what are common things people would get a COA for? Also, is there any way a COA can be given for special circumstances for someone to work (on a visa that would not typically allow them to, ie - in UK on fiance visa and UK fiance just laid off, etc.)?

I'm not getting one, just curious what they're mainly used for.

I got a COA.  It's permission to get married, not to change status.  It's for people (non EEA citizens) who are in the UK and want to get married, but do not have a fiance or marriage-visit visa.  I was here on a work visa, and had to get a COA before we could give notice to the registrar to get married.  It has nothing to do with changing your actual visa status.

It has nothing to do with changing your actual visa status.

I must have mis-understood what I was reading on Transpondia.

Thanks, springhaze.