Enabling archivelog mode question

I am enabling archivelog mode on one of the databases. My paln is to do the following:

1)shutdown the database

2)startup mount

3)alter database archivelog

4)alter database open

5) shutdown

6) startup

Then I was looking through oracle documentation and found

alter system set log_archive_start=TRUE scope=spfile;

can someone tell me what exactly does it do and whether I need it