i was reviewing my details for metamask
i noticed i have metamask seed phrase but not private key
so is private key important , what is it used for
i can recover my account with seed phrase so why private key
also how can i get key back , i am sure there would be some way
The seed is to generate pairs of private keys and associated addresses. One seed can generate an infinite number of addresses and each one of these addresses has its own unique private key. You can restore all of them if you want but as you have the seed then you are good to go.

Yes it's possible to import the private key for each address in your metamask.