Thanks for the heads up, I got S on the ogre, but only C on the troll. probably would have deleted and tried to obtain again if it wasn't for this comment

As other comments stated, thoses knowledges are only available once per account, so please do use everything you can in order to achieve high rank knowledge(elixir,pets,event bonusses,pearl outfit...).If somehow you get a low grade, well you are not alone sadly.In order to change this system i made a feedback on the official forum , you can got there and support the feedback by adding one upvote , and maybe one day PA will allow us to "reroll" this knowledge.

NB:this is the case for some knowledges and i extend the feedback as soon as i discover another knowledge in that case; already made a ticket for that and the GM just told me to go to feedback section. If in any case you deleted the knowledge, try to ask for a new one in a ticket as its your sole solution(which i can't guarantee).
Why getting high grade is important: Decreased damage received when the target hit you; Deal extra damage to the target (which can be interessting in order to achieve DPS rank against world bosses);Extra drop rate (even if it may or may not work against world bosses);Ecology point system;personal completion.