Given the two facts:
- According to wikipedia, the IATA represent a little more than 80% of available seat kilometers. It makes sense as some airlines are not IATA members.
- Under each region over the world, an organisation is dedicated to make sure civilian aviation is safe, following ICAO's standards (EASA in Europe, FAA in US, ...)
As I understand, the IATA has its own certification program (IOSA). It seems to me that it doesn't make sense: it doesn't cover all commercial aviation (some low cost carrier are not part of the IATA and still represent a significant part of the traffic), and its purpose is the same as other civilian organisation (FAA, EASA,...) acting under the ICAO's rules.
I'm quite sure I'm missing something, but is the IOSA useful as it does not cover significant airlines and its purpose seems to be covered by other organisations?