When do we use the uninstall meter function on edit meter details?

uninstall meter frees up the serial, so that you can add the meter again.
So if you take out the meter at one place, and install it in a new place, you can uninstall it from the first place.
This will essentially put _OLD behind the serial (e.g. serial 123456789 will become 123456789_OLD), allowing you to add the serial again for another customer.