Additional to previous answers. It's needed not only to protect from rogue, also it's required because of protocol design. Let me explain:
Nobody could be sure that somewhere else there is no other bitcoin-branch, larger, when it is available in that part of the network.
The Bitcoin protocol says that the largest branch is always true. If someone shows a chain of blocks larger than mine (which means currently in my node), I believe it and use it. Imagine a situation where someone from Canada made a block and announced it on the network. At the same time, someone in China also made a block and announced. Due to network delays, the Chinese part of the network learns about the existence of chinese block before canadian block, and opposite situation in Canada. Since the branches are equal in length, it is impossible to tell which branch will be truth and here the race begins: who will make the next block faster.
And there possibility to mine without announcement for a while..
In simple way: much more confirmations, much more proof-of-work over transaction
(Here is link with latest orphaned-blocks:
https://blockchain.info/orphaned-blocks)