- creating test wallets where you can more readily automate checking addresses for funds later on

- deterministic priv/pub key generation from the master for use as ephemeral account (nostr private DMs)

- setting up children's accounts where you are effectively an escrow holder

One seed to rule them all

Yes, BIP85 for being the uncle Jim of the family is extremely useful - and for gift sats. No more lost gift sats ever again.