Ethereum (ETH) is the native token for the Ethereum Chain. In order to swap ETH as an ERC-20 token, it needs to be wrapped. The wrapped version of ETH is Wrapped Ethereum (WETH).
Swaps on the Uniswap Protocol can start and end with ETH. However, during the swap, all ETH is wrapped into WETH. This is because in order for tokens to be swappable on the Uniswap Protocol, they must be an ERC-20 token. However, ETH was built before the ERC-20 standard.
This is why WETH was created. WETH is an ERC-20 token and is the current accepted standard. It can be used to swap with other ERC-20 tokens on the Uniswap Protocol.
When removing liquidity, using WETH as the output token over ETH is cost-efficient.