Vbucket vs indential node

I have read couchbase documentation where i have seen nodes are identical mean suppose i have 3 nodes. So data in bucket (e.g demo) same or identical in each node . Now when i read vbucket it says data evenly distributes across nodes and when i want to get any document , function process and get hash (i.e nothing but vbucket id something like that) and it’s identify which node have such document.

So i am confused if data identical to each node why vbucket required ?

Thank you