The same scaled system was used in Pride of Nations - which makes sense as the same pair of designers are behind both
Without going too far into detail, it looks like every nation in Empires is playable, including those that will spawn in game (Galatians, Parthians, others depending) but in truth about 10-15 are going to be feasible/interesting. So yes, you can play as some bunch of Steppe nomads, no doubt have great fun but ...
All use the same legacy system to derive their final score/win. But there are many routes to gain/lose legacy, not just conquest. And you can get legacy for being a long lived now decadent/old state - after all the Roman Empire has 'legacy' even if it collapsed. So if you build a small rich cultured state you may well gain a lot of legacy, perhaps more than rampaging your way to regional or global dominance. So different playstyles and options open up for different states so while all use the same scoring each have some pretty unique means of running up a decent legacy score.
And an ill-thought out rampage will cost you more than you gain, so simply constructing an ephemeral Empire stretching over poor provinces may not give you much legacy.