Gareth
The RAF recorded EEGs to be kept as a baseline reference in the pilot's files. If that pilot subsequently suffered a head injury playing sport, in a car smash, or by falling off a ladder, any EEG taken as part of the clinical assessment of the head injury may reveal some "abnormalities". The question then arises if these abnormalities were present before the accident, ie are a normal variation.
By looking at an earlier EEG the medics could confirm if the "abnormality" was present before the accident, and if so then a pilot trained for 3 million UKP could be returned to flying. Saving one pilot's career for the cost of about 150 x 70 UKP per year made economic sense then.
If something grossly abnormal was noted on the initial (baseline)EEG, then further neurological studies were carried out before selection was approved or rejected. This was not the prime reason for doing EEGs though.
Arguably the civilian Loss of Licence or PHI insurers could demand a baseline EEG in order to cover a pilot for the same reasons the RAF did. Perhaps there is a case for pilots getting one done privately for storage in case they get a whack on the head later on in their careers and they are in the sorry position where a group of neurologists are frowning over a few wiggles in the voltages, with little option other than pull his licence, but if there was hard proof the EEG hadn't changed since the accident they wouldn't have to. Food for thought.