TRY/CATCH statements are used to rollback a transaction if an error occurs in processing. The COMMIT goes at the end of the TRY and the ROLLBACK goes inside the CATCH. Without them, the transaction is left open if an error occurs during processing. This open transaction will continue to hold all the locks it established until the the connection is disconnected, the transaction is rolled back manually, or the process becomes a deadlock victim. In the case of a long running process that doesn't have a command timeout, that's essentially forever - at least until the 'all knowing' DBA terminates 'mysterious hung processes' as part of his daily routine.