Pathauto Module: Can't understand "Update Action" Options

Pathauto Module: "Update" Option

Hi.
In Home | administer | settings | Pathauto
Update action:
- Do nothing, leaving the old alias intact
- Create a new alias in addition to the old alias
- Create a new alias, replacing the old one

Q:
What're the difference between the first one and the second one?
It seems to me:
1st) No update will be made. Thus the old alias is still intact.
2nd) Update will be made, but old alias is kept

However there are "Bulk generate ??? (eg index, blog) aliases" option already.
They can control whether old alias is updated or not.
This makes "Update Action - 1st Option" as redundant.

Did I miss or misinterpret anything?

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I'm using Drupal 4.6.3
Thanks for people who are eager to help me :D