Ethereum incentivizes miners to include a list of uncles or ommers when a block is mined, up to to a certain limit. Although in Bitcoin, if a block is mined at the same height, or if a block contains no transaction, it is considered useless; this is not the case with Ethereum. The main reason to include uncles and have them as an essential part of the Ethereum blockchain is that they decrease the chance of an attack occurring by 51%, because they discourage centralization.