Is there any part of the island of Great Britain which is not in England & Wales or Scotland?
The "draftsman is playing it safe" insofar as he had no actual reason for introducing a definition of a phrase that is not contested or confusing.
The reason for the revised definition of "United Kingdom" is Irish independence
That provision of the Interpretation Act 1978 comes with a bracketed reference [12th April 1927], which means that it applies only to legislation after that date. For earlier Acts, the reader has to guess what might be meant based on context - depending on the date, the subject matter, and the care taken in drafting, "United Kingdom" might mean both Great Britain and Ireland, or just Great Britain. The precise date is because that is when the Royal and Parliamentary Titles Act 1927 came into effect, which included as its section 2(2) the words
In every Act passed and public document issued after the passing of this Act the expression "United Kingdom" shall, unless the context otherwise requires, mean Great Britain and Northern Ireland.
The 1978 Act repealed the words "Act passed and", so that the remaining provision of the 1927 Act is about other public documents, and the provision about legislation was consolidated into the 1978 Act. Similar manouevers were done with other interpretive provisions, as a tidying-up exercise, so 1978 didn't bring a substantive change in the meaning of "United Kingdom". That already happened in 1927.
The proximate reason was of course to recognize that the formation of the Irish Free State (as it was known then) meant that continuing references to the whole of Ireland as being part of the UK would be misleading.
Interpretation Acts are only trying to make useful definitions for legislative purposes; they are not dictionaries of every possible word
The Interpretation Act 1978 does not seek to define everything. Which terms get defined is a matter of overall convenience. Sometimes it is about avoiding repetition of the same (or worse, slightly different!) words in lots of different Acts, like:
"Writing" includes typing, printing, lithography, photography and other modes of representing or reproducing words in a visible form, and expressions referring to writing are construed accordingly.
It would be awkward if every statute that involved writing had to repeat the list of alternatives, and confusing consequences might arise if one of those instances happened to omit "printing".
Other definitions may be about brevity, like the definition of "Privy Council" as "the Lords and others of Her Majesty’s Most Honourable Privy Council". Note that this does not itself define what Her Majesty's Most Honourable Privy Council is! It does not need to, because we the readers know the answer; moreover, it's a body that exists by virtue of the royal prerogative and not any statute.
The 1978 Act derives from recommendations of the Law Commission (see in Command Paper 7235) on updating its predecessor, the Interpretation Act 1889. The Commissions saw several instances where there was a need to tweak the statutory language, but they did not see fit to add a lot of new general definitions, especially for terms like "Great Britain" which are already fully understood without difficulty.