The Instrument Rating (Restricted) (previously known as the IMC rating) permits the pilot to fly in a wider range of weather conditions, such as cloud and poor visibility. It improves one's knowledge, experience and utility as a pilot. It is a great way of building one's skills and confidence, and is also likely to increase your cross country flying time given that it is often bright on the east coast and overcast in the west, or vice-versa.
If you hold an UK PPL you can apply for this limited form of Instrument Rating. This rating is not recognised outside the UK but is a lot simpler to obtain than the full IR.