Ommer Blocks are valid blocks that were not included in the main chain because they were defeated by another block in the mining competition. This can happen when two miners complete blocks almost simultaneously, but only one will be included in the main blockchain.

On the Ethereum PoW network, Ommer Blocks are still incentivized in the form of a smaller block reward than the main block. This is in contrast to orphan blocks in Bitcoin which are not incentivized in any way. 

The aim is to reduce wasted energy in the mining process and improve network security while still considering the contribution of blocks that almost made it to the main chain.

Ommer Blocks provide several benefits in Ethereum's PoW system before the transition to PoS, including :

• Prevent Wasted Energy in Mining System

In a PoW blockchain network, a lot of computing power is wasted because only one block is selected to enter the main chain. By incentivizing Ommer Blocks, Ethereum reduces this energy waste while still rewarding miners who have worked hard to complete blocks.

• Providing Additional Incentives for Miners

Unlike Bitcoin which ignores orphan blocks, Ethereum encourages the inclusion of Ommer Blocks by providing rewards. This makes the network fairer and allows more small miners to still participate in the ecosystem.

• Improving Network Security

By considering Ommer Blocks, Ethereum strengthens its network's resilience to attacks. The existence of Ommer Blocks reduces the likelihood of attacks by 51% because large miners cannot completely dominate the network.