The Coil unit is typically skid or truck mounted and used to...

The Coil unit is typically skid or truck mounted and used to unload the frac fluids from the wells.
It is run in the well from a coil, (can be different diameters, lets say 2") and can have different tools on the end of it.
In this case probably a nozzle for releasing nitrogen at depth to safely lift the heavy frac fluids out of the well until naturally produced gas takes over.

There could be many reasons to replace or change out the unit.
For instance it could of failed a pre job pressure test prior to running in the hole.
This is not uncommon as the tubing wears and ultimately gets holes in it. At this point the complete drum is usually changed out.