The main Purpose is to give users automatically the possible advancing their allowed Settings the more posts they make.
For example you could define that a "newbie" with 10 posts or less is not allowed to include html tags in a signature, but you want to have an "Advanced Member" with more than 10 posts allowed to use html tags.
If you don't wont to use Ranks its no problem, you only need the Roles which use the same Settings.
Roles are manually assigned (Except the Roles which have the "IsStart" Flag) and Ranks can override the Settings if the are set up as "Is Ladder Group" and the user is automatically assigned to the Rank when he/she reached x amount of post.