What is this diffrence... How does it change the game? Excuse my naivity.
You have to see this from the perspective of clans.
Spartan clans are usually either UNSC, ONi, sniper, USA or other military clans with some occasional Griffball, assassinations and other clans.
Sangheili clans are primarily fleets, with a ranking structure based on the lore, with armours usually based on the lore, with people clinging to the lore. They also happen to usually be meritocracies and not democracies. Sangheili clans thrived for 8 years until 1 day Sangheili were removed from MP because a small portion (compared to the anti-sprint crowd) of MLG pros and people could not aim 1 cm higher (when the Elites were taller) based on what fans believe since there as never been an official statement in the past 3 years as to why. A Sangheili clan could face another Sangheili clan simply because:
they supported humans
they had different views on worshiping the Forerunners and their tech
they had an incompetent leader.
Sangheili clans usually formed alliances known as Covenants, forged covie themed maps, attacked UNSC clans, made ships on Forge, had cannon determine clan policy. A Spartan non lore based clan, which is part of the vast majority of Spartan clans, was a generic video game clan where people usually have to change their name or 1v1 their leader. Most Sangheili clans functioned completely differently. Most of the members where anti-UNSC or disliked humanity in Halo or at least preferred the Sangheili to the humans. A Sangheili clan would send an Ossoona to infiltrate an enemy clan, steal their forged maps, or 1v1 clan leaders in banshee fights as the High command of 1 or both clans watched. A Sangheili clan usually had a website/forum and a way for members to interact and create stuff together. Some clans even had websites for their alliances. There were coups assisted or not by other clans, clan meeting on Sangheili themed forged buildings or cities. There was also a ton of lore discussion as these clans combined both lore and campaign. In a Sangheili clan you had your character and your story. A spartan sniper clan set on noscoping does not have this. A Spartan lore clan doesn't also have the part of the lore that Sangheili players like. I dislike the UNSC and ONI with a fervent passion and no matter how many times I had been asked to join a Spartan clan since halo 3 I refused just because they were humans. If I wanna play as a generic supersoldier I can do that in any videogame. Halo was different. I was the alien. I was the Tier 2 alien reking UNSC newbies with plasma fire and helping the covenant cause or that of a splinter faction. I was the bad guy but I had a reason and that reason was based on what I liked in halo's lore. If I wanna be an anticapitalist Innie/or a Marine I can be in every game. Halo gave me more. I was the alien. I don't like playing as a human in video games. In Mass Effect I love the aliens, regardless of race. Just something that is not human or human with a prosthetic head. That's it. You can also see this the other way around. Imagine a halo game with only Sangheili and no spartans. There would a total havok and armageddon of complaints. It's just that Spartan players always get their playable model because it is human and that's what most people want to play as