CPF is a retirement fund that the govt forces on everybody. To ensure its people age well, the basic thing the govt can do is to help us have enough money by then. You wanna use the money, you follow its terms and conditions. If you can pay w cash instead of CPF then you pay less when it comes to the 2.5% lor. Call this a paternalistic state or whatever, but not everybody is financially savvy enough for the govt to allow much flexibility. If you're good enough at investing then go earn more than 2.5% so that you have no worries repaying. If you borrow from a bank, you need to pay interest. That is why you pay hdb the 2.6% interest. If you use your CPF money, you are sacrificing the interest it otherwise earns. So it makes sense that when you return the money, you include the interest. Because that makes up your retirement funds anyway. Borrowing money from your future self means you need to pay your future self the interest he/she deserves.