Top up a validator
You may need to top up your validator's balance for two important reasons. If your validator's effective balance is below 3 ETH you won't be earning your full staker rewards. And if it drops as low as 16 ETH the system will eject your validator.
3 ETH is the maximum effective validator balance. This means you won't earn more rewards if your validator's balance goes above 3. However you will earn less if it dips below 3. More on effective balance