Mining hashrate is a key security metric. The more hashing (computing) power in the network, the greater its security and its overall resistance to attack. Although Ethereum’s exact hashing power is unknown, it is possible to estimate it from the number of blocks being mined and the current block difficulty. Using data on who mined how many blocks, we can roughly estimate pools' hashrates.
Note: on 15 September 2022, Ethereum changed their consensus model from Proof-of-Work to Proof-of-Stake and this metric became obsolete.