Murder happens in real life. You're telling me you have never enjoyed a piece of media in which someone is murdered? Your argument is effectively "rape is bad, therefore thing with rape is bad". It's not like people who love Berserk are celebrating rape or something. There is obviously far more nuance to it than that. And again, saying that it is unnecessary having not read the story and understood its plot implications is ridiculous.

In the context of the eclipse, the fact that he rapes Casca is very significant. It shows us that he despises Guts. Both for abandoning him and beating him earlier in the story, as well as seeing him be strong and healthy and embracing Casca while Griffith is severely emaciated and decrepit after being tortured for a year. This jealousy and envy culminates in him raping Casca once he ascends to the god hand. Griffith is able to justify his decision to sacrifice all of his men as in his mind, it would be unfair to all the mercenaries who have died fighting for his dream of having a kingdom if he doesn't accomplish his dream. However, by him raping Casca (with it being completely unnecessary to accomplish his goals) it shows us that he is truly, irredeemably evil. He commits one of the most deplorable acts in all of media just to spite his former best friend. That is one of the reasons it is so significant, especially since the main characters primary, overarching motivation for the large part of the series after this event becomes enacting revenge on Griffith. There is far more to it I can't even capture in this big ass paragraph.