Unavoidable, inevitable.
Required, essential, whether logically inescapable or needed in order to achieve a desired result or avoid some penalty.
Extra characters such as spaces added to a record to fill it out to a fixed length.
Soft filling material used in cushions etc.
Extraneous text added to a message for the purpose of concealing its beginning, ending, or length.
Anything of little value used to fill up space.