@gum_rummy
It's really necessary, for two reasons.
One, Naofumi is paranoid as fuck (see page 18, where he says, "Everything was to mislead me... to ridicule me."). In that moment, he's probably thinking that Raphtalia too wanted to deceive him, and set up the duel in order to be freed. Raphtalia knows that he's innocent and didn't rape that bitch, but realises that he's incapable of trusting anyone, as he was doubting even her (page 11, "[...] And yeah, let's add Raphtalia-chan to that equation, too. Every single one of them has made a fool of me.") . So, she's willing to put the curse back on, for Naofumi's peace of mind (pages 23-24, "Do you still not believe what I'm saying? Then please place a curse upon me"). It's pointless, because he's not going to boss her around, nor she'll betray him, but it's what he needs right now.
Secondly, it seems like the slave mark is needed for her to be in his party: once it was removed, she was no longer in it. This mechanic is kinda sketchy, though, since she would follow him, mark or not, and I don't see how that isn't enough for her to be in his party.
It also has a symbolical meaning. Much like wedding rings are a symbol of marital vows, that mark is a symbol of her dedication to him. He saved her life, and gave her a purpose to live for: she owes her existence to him, and as such, she is, in a way, a "slave" to her gratitude and loyalty. It represents her deepest trust in him, as if to say, "I give you absolute power over me, knowing you won't hurt me". It's meaningful because it's pointless for her to have it, since it won't be used.
Finally, from a narrative viewpoint, it's an useful narrative device to create tension. Would he
ever use the mark? Of course, we expect him not to, ever. But,
he could. He's not an idealistic knight in shining armour, he's willing to resort to underhanded tactics (but mostly out of spite for this world that keeps on beating him down). And he's literally picturing himself as a villain in page 10, he came real close to snapping, if it wasn't for Raphtalia. Could he ever succumb to his darkest, most destructive impulses, and end up using the mark? Would Raphtalia forgive him? It's interesting!
It may be gross to you, but I think that Raphtalia wears her mark with pride, not because she's proud of being a slave, but because she's proud of her master. I think it's most evident in pages 14-15, where she roasts Motoyasu to a crisp by telling him that he's just a vainglorious piece of shit, who'd never take in a filthy, starved, diseased slave on the brink of death, and nurse them back to health, like Naofumi did. You could argue he picked her because she was cheap, but if that was the case, why does he treat her in such a humane manner?
(Shit why the fuck am I studying law, I should've picked literature instead...)
