1) What is the purpose of having VALID_FOR attribute in LOG_ARCHIVE_DEST_n parameter?
If you mention the valid_for attribute in LOG_ARCHIVE_DEST_n which refers the destination kind of redo it uses and the database_role.
2) If i don't set VALID_FOR attribute in LOG_ARCHIVE_DEST_n parameter, does it have any adverse repercussions?
VALID_FOR is an optional, it is not mandatory parameter either. But there are several settings for Physical , Logical and primary role databases and if you perform Switchover(role transitions) there will be no issues later on.