The pledge exchange is basically nothing more than an exchange of the real estate in which the construction financing remains at the agreed terms.
When this is an option? For example, when you own a home and have to move due to work. Then it is clear: You need a new property. If you want to sell the old property, you can talk to your bank about continuing the current loan - just with the new property.
Just keep in mind: The bank does not have to agree to a pledge exchange. And in all likelihood they will not do so if it puts them at a disadvantage. But if the new property is at least of equal value, the chances are good.
Otherwise, the alternative would be to pay off the loan early - with the money you got from the sale - and pay a repayment loan.
Although the exchange of pledges is not free, the costs are usually lower.