Decentralisation of the Cardano ecosystem is important because it helps the network maintain operational resilience against denial of service attacks and other forms of network and service outages; for example the Dyn DNS outage that took a large amount of the internet offline a few years back.
It also helps ensure that the network is secure to 51% attacks by malicious actors who wish to subvert the validity of the block chain record.
At this point in it’s evolution this graph shows the impressive decentralisation that the Cardano ecosystem has already achieved. You can see the live searchable graph at PoolTool.io.