Front-end developers with solidity knowledge are in high demand because they are able to develop user-friendly and secure interfaces for blockchain applications. Solidity is a programming language used to create smart contracts on the Ethereum blockchain, and front-end developers with knowledge of this language are able to create user interfaces that allow users to interact with the blockchain in a secure and efficient way. Companies across various industries are looking for developers with this skill set, and the demand is expected to continue increasing as the blockchain industry matures.

Frontend devs do not require rich knowledge in solidity. But they have to know the basics like "how to call this function from the frontend", "how to send parameters to solidity function and receive the result", "how to send enough gas to call this function"
If frontend devs are able to understand the logic of smart contract, they can ask smart contract devs to adjust function. eg: avoid sending unnecessary params, get easy-to-use result, etc.