Possibles, including some already mentioned;

He's never had a licence and is now going legal.
He's come out of being banned and has to retake his test.
He's taking a refresher course, something everyone should do after 7 to 10 years.
He's going for an advanced test of some sort.
He's training to be an Instructor.
He's going from Auto to manual licence.
He's on a foreign licence and only just found out (or has been caught) he should have taken a U.K test.
He's been on a driving assessment with his company and failed and has been asked to take lessons.
His mate is an Instructor and is giving him a lift home.
.