If Riven had killed Swain in that fight at the party, there would have been a political uproar.
Noxus would be after Riven like hounds, the League would be unable to help her, and Demacia wouldn't be able to help her without being in conflict with the League.
She would become hunted with no one to help her for the rest of her life.
Instead, she made it an April fools joke.
She destroyed countless Noxian soldiers, overpowered Swain in his ultimate form, and had him down to the very wire, when she played it off as an April Fools joke, and bounced away.
She worked within League's boundaries, and revealed to both Noxus and the entire world just how strong she was and shamed Swain in front of his own people.
So not only did she prove she's one of the most powerful beings in all of Runeterra, she did it without any repercussions, like a genius.
That suit is a symbol of that incident, and she wears it to remind people of that.
Sounds like SOMEONE has been reading up on their Renekton Bot fanfiction...
Looks like I'm off the the fanfiction section.