Your seed phrase is what that you can use to access your bitcoin on your wallet and to spend the coin. Many people are sending email that they should enter their seed phrase on a site. If they enter the seed phrase their, the bitcoin will be sent to another bitcoin address by the scammer.
Do not backup your seed phrase online because scammers can get to it online.
Why will anyone send there see phrase to someone else, especially sending it to someone online. Your seed phrase is your security to protect your coin.
It will be foolish of anyone to fall for this scam. It's like giving someone access to your bank account login, they will definitely clear your money, same applies to when you give someone else your seed phrase. You have already giving the person access to your coin and they can do whatever they like with it. So it's advisable not go give anyone such vital details.