User blog comment:Mineko Charat Lucky/League and the Institute has nothing to do with League of Legends anymore(?)/@comment-7782121-20130815070803

Out-of-universe theory: The present writing team or direction of the writing team is emphasising champions as individuals, though things like homelands and locations are still mentioned. Maybe they're phasing in the city-states to have a bigger role, hence the Freljord launch event. So the purpose is still there (fight in arenas, avoid wars), but the middlemen aren't as prominent.

In-universe theories, though, as to why they'd willingly barter their skills:

1. Perhaps like Vayne, champions who could be independent join the League (or whoever runs it) to keep an eye on their enemies or dangerous individuals. If nothing else, spending time on hundreds of battles can give you an idea on what your enemy could do once you're out of the Fields of Justice, whether they were on your team or not. The reverse might hold true: Maybe some seek asylum from their nemesis, knowing the League provides, if not total immunity, a level of protection from an all-out battle to the death.

2. Maybe it's a Morgana-Kayle thing. Morgana may want Kayle dead, but she enjoys the prospect of killing her an infinity amount of times. Rengar may want Kha'Zix as his ultimate trophy (maybe he's even tried like Kassadin and Malzahar clashing), but he certainly enjoys a good hunt enough to participate willingly. The League is an outlet for their frustrations at any stalemates or unresolved issues, a practice run for the real thing. Or otherwise, a long-term consolation prize or excuse for violnce. Aatrox at the least definitely falls into this category through conflict for its own sake.

3. Could be an influence/assistance thing. Nami's quest may continue outside the League, but her presence in it gives her access to accumulated knowledge fom both summoners and Champions from around the world. Heck, her status as a Champion grants her much easier access to the Howling Abyss where they were apparently 'fresh out of moonstones' (I blame Diana :P). A similar but more detailed example can be found in Zilean, though his case seems like it needs more help than Nami's, so he might not count as independent.

4. Lastly, could be a sense of duty to the homeland, like when Hecarim demanded entrance into the roster. That sense of duty might vary from one place to the next: some are based on profit, others loyalty, and some for egos. Perhaps it's a self-fulfilling thing: You see champions in a rival state gathering and joining the League, so to make sure your side remains competitive, you join. Then others see the two nations getting more champions, so their countrymen get involved to protect their interests.