The Sync button appears at the top right of the contract page after the contract has been deployed (published) to the selected blockchain network.
The button lets manual requests for fetching the contract transaction data through the applicable explorer API. Then, if its actual state differs from the one displayed on the page, applicable updates will be reflected on the contract page.
This gives users better control over the interface. It also ensures that the transaction status (and the contract state) is clear to prevent overriding the transaction in progress.