The "need" is that Bitcoin can't work if all the people who have chosen to agree with and enforce the current ruleset suddenly decide not to agree anymore.  It's a simple need, but it generally outweighs everything else. 

If you ever decide to disagree with the current ruleset and actively enforce a new one, you have to leave this network and start building a new blockchain of your own.  Unless you can populate that new network with a sufficient number of users, miners, full nodes, retailers who accept the coin, etc, your new blockchain will be incredibly weak and would likely die very quickly.