It was suggested to be by a senior barrister who I respect (Mark Herbert QC).
Legal and human interest, the challenge of analysing a problem plus the satisfaction of sometimes being able to help provide someone with a solution to a difficult problem or at least to ensure that no further time is used up when the case is hopeless.
No concerns with money or time, but all the other satisfactions of engaging in legal work.
My experience is that most lawyers go into the profession to help other people. Pro bono work is an excellent way of doing this whilst exercising your legal skills, and can have incidental benefits in enlarging one’s experience and providing a feeling of satisfaction.