I have done this a few times, I found it a good experience. The main thing is that the junior doctors are tested on their communication skills, and on how they ask questions and listen to what is being told to them by the 'patient'. Then how to feed back to the patient and the examiner what he/she has learned, with an explanation and plan for treatment or further investigations that may be needed.
This becomes more important as the junior doctors progress in their training through the various parts of their membership colleges -MRCP; MRCS: MRCGP; MRCOG are a few. The exams for these colleges have a Part 1 and a Part 2, and some have a Part 3, and the parts have a timeframe to be completed in. The colleges also have an F (fellowship) along with the M (membership), and there is pressure to get the exams done along with the written elements. The further along the career pathway the more difficult the examinations are.
Some consultants (physician or surgeon) will have patients on their caseload who have very interesting or rare illnesses and conditions, not all that are 'visible', who agree to be in a 'pool' of people who will help out at these examinations. The examinees are then tested on what questions to ask to find a diagnosis. The real patients in these circumstances are greatly valued and everyone is hugely grateful to them. It is understood that the examination day is busy and tiring, and that these wonderful people can say when they have had enough. Many of the consultants will reimburse their patients - these consultants are most likely to be the examiners too.