Monster Knowledge is helpful in more than one way as it is not only showing you your enemies detailed HP bar and increasing your maximum energy. Monster Knowledge is also granting additional benefits while fighting your foe:
Increases the Chance for Rare Items
Increasing your damage even further
Increasing your damage
Reduce the damage taken
Detailed HP Bar
Ranks are downwards compatible which means you gain all the benefits of the lower ranks – e.g. as A Rank you gain the increased damage and the reduced damage taken in combination with the detailed HP bar.
If you do not have any Knowledge about from your target you can still “read” the HP Bar depending on the color of it.
Knowledge & the Cashshop
While gaining Knowledge in general and the Ranks in specific are completly RNG – you can basically S Rank on your first kill ever – there are two items from the Cashshop now only increasing your chance to gain Knowledge but to increase your chance on getting higher Rank Knowledge.
Both items costs 500 Pearls each and take up the costume slot (Eye / Ear).
Do you recommend those items?
On this I can only give you a simple and clear yes – at least your main character should use those two items, the increased speed is really noticeable and in combination with the higher chance on S Rank knowledge you will save so much time grinding for the S Rank and then for the actual item you are farming for e.g. the Ogre Ring.
For those of you that don’t want to spend money of those items: I have good news! For a total of 3000 hours of playtime (125 days worth of playtime) you will be gifted with glasses that increase your chance on getting knowledge.