The "Sync" button allows manual synchronization of transaction data between a blockchain network and user interface. Syncing can be used to update the current state of a contract manually in case of miscommunication due to connection loss or accidental exit, for example.
The Sync button appears at the top right of the contract page after the contract has been deployed (published) to the selected blockchain network.
Note that there is no transaction record in the network to sync before the contract is deployed (published) to it.
The Sync feature has been added to ensure that the user's contract page (from where the contract is managed through contract actions) and the blockchain network (where the contract resides) display the same contract state at all times.
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.