with thanks to davey23uk, what i understood is changes are always written to redo log. if DB in archive mode, on log switch archive is generated, if in no archive mode, changes are lost the moment redo log group is used again.

During hot backup, changes are written in datafile and recorded in redo logs.
then why the archive log mode is required (not concerning to restore and recover). (i understood db should be in archive log mode, what i want to know is what's happening internally during hot backup, such that archive log mode is required)

with regards