In simple words: the recovery phrase is used by the wallet to calculate a master private key.
This master private key is used to derive actual private keys. However, this process isn't 1-to-1. 1 Master private key can be used to generate an infinite amount of series of private keys depending on the derivation path.
It's actually a bit more difficult than this, but this is the general idear behind seed phrases, xprv, private keys and derivation paths.

This is why we need to know this derivation path. If you enter the wrong path, a completely "useless" series of private keys will get derived. There are some standards as to which derivation paths to use, but abra already said they use custom derivation paths.