Withrdawing bitcoin from web-based exchange wallet to local wallet. My wallet passphrase is requested- is this necessary?
NEVER share your seed pass phrase/mnemonic. Anyone that has it can steal your bitcoins, and there is NO reason a web-wallet would need it in order to transfer your funds. 

If the web-wallet is requesting a password in order to withdraw funds, it is most likely the password to log in to your web wallet account, or a secondary password you set up to spend funds from your web wallet account.