A seed phrase can derive both the public and private keys for Bitcoin addresses, as well as Nostr's npub and nsec. Npub and nsec can be understood as public and private keys in a customized format, which can be converted between the two formats. 

 I see. I wonder if there's a direct, straightforward application for that though. 

 To our knowledge, there are currently no Nostr clients that reuse npub as a Bitcoin address. There are already clients that use a seed phrase, such as Keychat.