Is it possible to use Transaction control Statements such a ROLLBACK or COMMIT in Database Trigger ? Why ?

Generally In triggers you can't use TCL commands.
But you can use TCL commands in Autonomous Triggers.
You can declare a trigger as Autonomous by providing PRAGMA AUTONOMOUS_TRANSACTION in the beginning of the trigger.
At a same time you have to end your trigger with commit/rollback.
You can use these type of triggers to maintain log details of a table.