Ranks function as a sort of leveling threshold in a system where levels don't exist.  They have requirements that, once reached, allow for a character to advance in power gaining significant bonuses and increasing the caps on their power levels across the board.

Many aspects of Essence skills, powers, and aura abilities take a rank modifier into account. 
The modifier is as follows:

Iron = 1
Bronze = 2
Silver = 4
Gold = 10
Diamond = 20

Iron rank grant a small advantages beyond what powers you managed to unlock.

When an essence binds to a core stat it increases the maximums of the resources associated with that stat.  

Strength (ST) - When an Essence Binds to Strength you gain the following benefits:

• Increase ST by +1 

• Maximum Hit Point advancement increases from +30% to +50% of the base amount.

Dexterity (DX) - When an Essence Binds to Dexterity you gain the following benefits:

• it increases Basic Speed by +0.5, 

• Maximum Basic Speed advancement increases from 2.0 to 2.5.

Intelligence (IQ) - When an Essence Binds to Intelligence you gain the following benefits:

• Increase Perception by +1, 

• Increase Will by +1

• Maximum Mana Point advancement increases from +30% to +50% of the base amount.

Health (HT) - When an Essence Binds to Health you gain the following benefits:

• Increases HT by +1 

• Maximum Fatigue Point advancement increases from +30% to +50% of the base amount.

Additionally when All 4 Essences are obtained the Essence magician gains the following:

• Damage Resistance:   DR all sources = Rank Modifier
  Reduce damage taken from all sources by an amount equal to your Rank modifier.

• Hard to Kill:   Level = Rank Modifier
  Bonus on Health rolls to resist dying equal to your Rank Modifier.

• Regeneration (Slow) - Recover 1 Hp 12 hours
  Regain 1 hit point every 12 hours, add your Rank Modifier to your Health rolls to recover from injuries and regain hitpoints.

Also, the following advantages may be purchased to further enhance your character once you have all 4 essences:
Ambidexterity - 5 pts
Brachiator - 5pts
Charisma - 5/level pts (max = Rank Modifier)
Common Sense - 10 pts
Fearlessness - 2/level pts  (max = Rank Modifier)
High Manual Dexterity - 5/level pts (max = Rank Modifier)
Night Vision - 1/level pts (max = Rank Modifier)
Silence - 5/level pts (max = Rank Modifier)