The incentive applies to nurses who may be tempted to go straight to the UK private sector, and to any HCP who would be tempted to move abroad for a few years but not necessarily for the rest of their lives.
I don't see why so many people are baffled by this proposal - it may not be right, effective, or fair, but it's perfectly manageable. Loads of women have a "pay your extra maternity pay back if you don't return to work" deal for example. No the government can't force you to work for them, but they can certainly bill you for extra tuition fees retrospectively if you do "the wrong thing", or write off your student debt if you do what they want.