The mottos are largely to help me with the design process, honestly. It's a valuable creative challenge coming up with them, because I have to think about what makes the character tick, what they're hoping for, which gives me a more detailed picture in my mind and should ideally make motivations and dialogue for them more consistent overall, rather than characters being interchangeable with their actions driven by an impersonal plot. Having them as a base also makes composing the characters' musical themes way easier, plus it has a nice feeling of enhanced meaning, tying things together, though I'd say that's a deep-down thing most players wouldn't notice. It certainly wouldn't be pushed in their faces, and many mottos might only ever be seen in the in-game Enyclopaedia (though others might occasionally come up in dialogue).