Understanding the 'public' item filter

I see that the ‘public’ item filter has the following rules listed:

  1. What does the sys_publicFilter do ? Why does it not have any parameters ?
  2. Is the order of the rules important ? Does the sys_filterByPublishableFlag[y,i] filter on the results of sys_publicFilter[] ?

I understand #2 returns items that are in Public or Draft state but I dont see how the two rules work together.
If #2 filters for Public and Draft anyways, what is the need for rule #1 ?