The answer is simple.
When you are using CPF to pay for the mortgage of your investment property that is rented out, you will receive cash from rent. In this way, the CPF withdrew has just been converted into cash by renting your property out.
Only use this method when you know how and have a plan for this extra income.
If you do not have plans to earn at least 3% per annum from this extra cash, I would recommend you to leave them in your CPF account for a 2.5% interest.