Every object in a database has a lifespan. Objects can be created, modified, and destroyed. In the RDBMS world this is achieved through SQL statements that don't always comply with the SQL standards. Some vendors provide more functionality and control than others, and compliance levels vary greatly among them. Virtually every object that could be created within a database could be altered or destroyed.
In most cases the ability to modify database objects is optional, that is, you can always use a set of alternative statements (like DROP and CREATE) to achieve the same results.
The ability to remove database objects is also very important. It releases space for physical objects that are no longer in use and can also be used to change objects that cannot be altered (by dropping and re-creating them).