At the start of the Babylon 5 series, the Council of Babylon 5 consists of five main ambassadors - human, Minbari, Centauri, Narn, and Vorlon - and a bunch of misfits from the League of Non-Aligned Worlds, such as the Drazi, Pakmara, and so on.
The humans are clearly entitled to one of the primary seats on the council: they built Babylon 5 and set this whole thing up. So are the Minbari: co-sponsors of Babylon 5, and a civilisation easily powerful enough to destroy humanity. So are the Centauri: they might not be in their prime any more, but they're at least the remnants of a great empire, as well as the first aliens to form relations with humans. The Vorlons are, as usual, a total mystery, but at least they're undoubtedly powerful.
So where do the Narns come in? They were a Centauri colony until recently (within living memory). What makes them more important and powerful than, say, the Drazi - enough to merit one of the five main seats on the Council, rather than a place on the benches with the less powerful races?