1. Context. Visible ranks help contextualize my play, and the play of my friends and enemies, such that I can understand -- roughly, with obvious limitations -- "how good" I am relative to those friends and enemies, and the wider ecosystem of Halo players. This helps me feel I am part of a large community of players competing in the 'sport' of Halo, and gives me a sense of how I am doing, how my friends are doing, and how we are improving, within that community of players.
2. Narrative. Visible ranks tell stories. They tell me I was good last month, but did even better this month. They tell me that I got better than the friend who used to school me at the game. They tell me I overcame an obstacle, or met a personal goal. They tell me that I pushed past a previous limit, or that I finally outpaced my peers. They tell me that I got better -- or that I got worse. They ground my Halo experience across a given game's lifespan, such that I can see not just that I was playing Halo for the last year of my life, but that across that time I was continuing to play Halo better, more skillfully, more beautifully than ever before.
3. Motivation. Ranks give me one more reason to play "one more game", because there's something to be earned, and that thing is earned through my own performance, not the sheer time I have spent playing. They give me a measure, however crude, of that performance, and meanwhile goad me to do better, to learn the game's systems more thoroughly, to improve myself. Related to the 'storytelling' aspect above, ranks also set up underdog battles, where every last one of my party grits their teeth and digs in because the pregame lobby told us we were outmatched. Those victories are sweeter than any other. Unanticipated defeats, on the other hand, are both humbling and inspiring.
4. Stakes. Ranks go up when you win, sure, but they also go down when you lose. Nothing in Halo 4 will ever go down. Not one thing. Not one thing will tell me I am not meeting expectations. And nothing will ever be lost. Once I unlock something, once I hit a target, it's mine forever. There is no risk in this, and so there is no genuine reward, or at least no scarcity of reward. Ranks, on the other hand, raise the stakes on every single game. There are days I wouldn't go into TS in Halo 3 because I was worried I might lose my 45 (hint: I'm not even that good, not that 'competitive', but I still love ranks). When I did go in, I took it seriously: I played the game as well as I could, and found teammates and enemies doing the same. Every game mattered, because losing mattered -- because losing had a cost. In Halo 4, I will be showered with announcements, medals, white text, XP, commendations, challenges, armour unlocks, weapon skins, and general praise almost to the point of suffocation. Not once, however, will the game dare to tell me I should have done better. Not once will it punish me for that failure. In sport, when everyone's a winner, no one is.