The purpose of Bitcoin HD (hierarchical deterministic wallets) is to enable a user to have limitless addresses without needing to go through the tedious process of backing up each address's private key - this greatly simplifies and encourages best practice to not re-use addresses.

But it was still difficult to write down an extended private key (error prone) - solved with BIP39, allowing users to write down WORDS from a protocol (2048 protocol words) which then are used to calculate the extended private key (a NUMBER) - the user won't need to write that down.

To see this in action, you can do this fun exercise of creating your own seed, and you might learn a lot...

armantheparman.com/dicev2/

https://twitter.com/parman_the/status/1523909150857908226?t=b5yr_TTaHpxf-LgOadrD3g&s=19