KKH subsidy

Hi mummies~ sorry I'm a first time mother please help me understand. I'm currently 15 weeks under KKH subsidy scheme. I don't really mind seeing different doctors for scans. But I've read some post where mummies will switch to private at KKH for delivery. May I know what's the common reason for such switch? Is it so that the doctor who will help you deliver is fixed (you know who he/she will be?) What are the other pros of doing this switch? Tyvm~