While ORM is great in simplifying our work with the database, it is still very useful to know SQL statements our application is throwing at the database. Specially when we are still developing the application.
One of the convenient features of Hibernate that is inherited by Grails, is that once you have defined your data model, it can create the tables for you automatically on start up of the application. The nice thing about this is since Hibernate/GORM is database agnostic, it means you don't need to know how to define the tables in different relational databases. Be it Oracle, DB2, or MySQL. If you change your connection property, and load the appropriate JDBC driver, it will know how to talk to the database and create the correct tables.