Quote from weewilly:
Here's an example. You short a stock and buy a call to protect the upside, so you are long the synthetic put. Sometime later, your broker notifies you that your short stock is now hard-to-borrow and/or has been placed on a high-volatility list, leaving you subject to an unwanted buy-in, excessive short stock fees, etc. Perhaps you simply want to free up margin for other purposes. But you still want an equivalent position.
You place an order to buy a conversion at the same strike as your long call. When filled, you have "converted" your synthetic put into a real put.