Basically you set a columns to unused if you are concerned about the time it would take to actually drop the column. The column is treaded as not being present. It will not show up in queries and the name off to column can be used to create a new column.
The only action you can perform on an unused column is to actually remove it, there is no way to reactivate it.
The difference between DROP COLUMN and SET UNUSED is that the first actually removed the column and performs any clean-up action needed and the second just marks it for disposal.
Marking Columns Unused
If you are concerned about the length of time it could take to drop column data from all of the rows in a large table, you can use the ALTER TABLE...SET UNUSED statement. This statement marks one or more columns as unused, but does not actually remove the target column data or restore the disk space occupied by these columns. However, a column that is marked as unused is not displayed in queries or data dictionary views, and its name is removed so that a new column can reuse that name. All constraints, indexes, and statistics defined on the column are also removed.
To mark the hiredate and mgr columns as unused, execute the following statement:
ALTER TABLE hr.admin_emp SET UNUSED (hiredate, mgr);
You can later remove columns that are marked as unused by issuing an ALTER TABLE...DROP UNUSED COLUMNS statement. Unused columns are also removed from the target table whenever an explicit drop of any particular column or columns of the table is issued.
The data dictionary views USER_UNUSED_COL_TABS, ALL_UNUSED_COL_TABS, or DBA_UNUSED_COL_TABS can be used to list all tables containing unused columns. The COUNT field shows the number of unused columns in the table.
SELECT * FROM DBA_UNUSED_COL_TABS;
OWNER TABLE_NAME COUNT
--------------------------- --------------------------- -----
HR ADMIN_EMP 2
For external tables, the SET UNUSED statement is transparently converted into an ALTER TABLE DROP COLUMN statement. Because external tables consist of metadata only in the database, the DROP COLUMN statement performs equivalently to the SET UNUSED statement.
Removing Unused Columns
The ALTER TABLE...DROP UNUSED COLUMNS statement is the only action allowed on unused columns. It physically removes unused columns from the table and reclaims disk space.
In the ALTER TABLE statement that follows, the optional clause CHECKPOINT is specified. This clause causes a checkpoint to be applied after processing the specified number of rows, in this case 250. Checkpointing cuts down on the amount of undo logs accumulated during the drop column operation to avoid a potential exhaustion of undo space.
ALTER TABLE hr.admin_emp DROP UNUSED COLUMNS CHECKPOINT 250;