In a real-world application, errors always appear. Syntax or even logical errors can be in the code, the database design might be incorrect, there might even be a bug in the database management system you are using. Even if everything works correctly, you might get an error because the users insert wrong data. With Transact-SQL error handling you can catch such user errors and decide what to do upon them. Typically, you want to log the errors, inform the users about the errors, and sometimes even correct them in the error handling code.
Error handling for user errors works on the statement level. If you send SQL Server a batch of two or more statements and the error is in the last statement, the previous statements ...
