The staker vault allows liquidity pool LP tokens to be staked to receive MERO governance tokens. The staker vault also supports some ERC20-like functions such as approve:
StakerVault.approve(address spender, uint256 amount) external
Approve staked tokens for spender.
spender: Address to approve tokens for.
amount: Amount to approve.
While the staker vault implmenets some ERC20-like functions. It is not an ERC20 token, and does not implement the standard.
The approval is used to allow Mero Actions to spend your LP tokens, while still accruing MERO rewards. Assets registered for actions still earn interest and MERO rewards.