Synchronous operation is a blocking operation as a process will wait until the operation completes. But an asynchronous operation is non-blocking as it only initiates the operation. An asynchronous operation allows more parallelism. Since the operation do not block the process, it can do some other computation while the operation is under progress.
A process flow is designed more as an asynchronous process and 'the submit' from a process button will initiate the business process, but the form need not wait for the business process to complete. Also, most processes take days to complete and hence it is pointless to wait for a process to complete.
A Microflow can be designed as synchronous process where the system will wait for the data to be returned from a Microflow or as an asynchronous process where the user can continue to work on the screen while the system will compute in the background and update screen with the result. When using microflow, user can use screen blocking feature to make the call a synchronous or an asynchronous operation.