Why do we need to have a centralised hierarchical authority in our communities instead of letting them organise themselves? I am not well informed about this, so I apologise if this is in the wrong place.
[Assuming you are talking about a human community/society in general]
I'm not sure that we need centralized authority, but that's just what you see developing in communities.
Authority comes with roles, duties and capabilities, so as with any other human endeavour, it pays to let certain people handle those roles and duties. This is the specialization that you see throughout developing communities.
There is not really much difference between asking/helping/assigning someone to (take on) the role of carpenter, policeman, garbage hauler or religious leader.
The incentives to go for specialization do not only come from the people wanting to take on a role, but the rest of the community benefits of having the services available and not having to do it themselves.
Then, if you want those specialized services available in a growing community, centralization seems inevitable: people pool their knowledge, your locksmith chooses to live in a city where he gets more customers, etc.
Your organic non-centralised communities usually turn into organic centralised communities. There is nothing artificial or non-organic about centralization, you can even look at a community as an organ.
So, to quote you, letting them organise themselves can easily turn into a centralised hierarchical authority, and that development is entirely organic. There is no contradiction here.