Others have gone over the benefits of the Arc generally. The reason why level 80 specifically, rather than say 75, or 90 has to do with a couple of things. First, the rewards per level go way down after 80 (from 10 - 60 it's 1% increase in bonus each level 60-80 is 0.5%, after 80 is 0.1%). Second, past 80 and you're on the wrong side of the profit zone curve, so even with donors giving a full 1.9x of each spot, it's still pretty expensive to level (and increasingly so every level). At 80, you get a 90% bonus (or 1.9x - hence 1.9). For these reasons, there seems to be a general consensus that the 1.9 times the reward is what a spot on any GB "should" cost, and players and guilds run 1.9 threads based on this. Sometimes a very generous high level player will give more than 1.9, but that's not nearly as common. Vendors of high level goods often quote the price in 'net over 1.9', meaning, they'll expect a full 1.9 donation, and then the agreed price of the goods on top of that. You want to power level your other GBs, where are you going to find the prints? Think you'll be seeing players post high level buildings in your guild's swap threads? No, they're leveling them in a 1.9 thread, with other level 80 Arc players taking the spots using their fp packs, which they will get back in full when the GB levels (usually very quickly) and end up swimming in prints. Without a lvl 80 Arc, you just can't keep up, and are locked out of the most profitable way to play the game.
