The IMC rating is for fixed wing pilots to allow descent in visibility less than 1800 m. It was intended as a sort of halfway house for PPLs who didn't really need the full IR, which was designed more for commercial airline pilots.
It only applies in the UK and was going to be scrapped in favour of the new EIR (Enroute IR) from EASA, but that does not allow descent, as the trip must start and end in VMC. The IMC rating has now been given a stay of execution (in the UK only) for the forseeable future, and the two complement each other nicely.
Neither apply to helicopter pilots.
Phil