Is trigger and sequence necessary for oracle table?

Hi all,

I am having table which contains around 50,000 of records.  Now these records are inserted simply without any trigger or sequence created for that particular oracle database table.

And one more thing I am not referencing it's serial number  anywhere in my code nor in query. So serial number in my table are as 1,2,4,8,11.....  (i.e. not in a proper way due to dml operations like delete.).

So is it necessary for me to have a insert before trigger on this table using sequence.??

My concern is that since that serial number is not needed, then why to give database more load by finding the last inserted serial no and insert the serial+1 number.