Do I need a separate seed for each wallet?
Keeping a new seed for every wallet you use leaves you with a pile of backups that, in practice, nobody guards well.
**With BIP-85, a single seed is enough to derive entropy for all your wallets: you don't need a separate backup for each.**
The problem with a separate seed per wallet
A new wallet asks you to write down its seed, a hardware wallet gives you another, and before you know it you have a stack of papers that should be stored in different places.
BIP-85 says it directly in its motivation: "Having multiple seeds is far from desirable, especially for those who rely on split key or redundancy backups in different geological locations".
That scenario becomes impossible to guard properly: either you leave copies unprotected, or you can't find the backup right when you need it.
How BIP-85 concentrates everything into a single seed
BIP-85 takes a single master seed and, from it, deterministically generates the entropy for each wallet you want to create.
It's not magic: it's a standard derivation. With the master seed and the path assigned to each wallet, you can regenerate any child seed when you need it.
That means your backup stops being an endless archive and becomes a single seed that you protect well.
What you actually write down
With BIP-85 you don't write down a seed per wallet. You write down the master seed and, if you want, the list of derivation paths you used for each.
That list is not a seed: it's a reference to rebuild each child seed from the master. You can store it in a less paranoid place, because without the master seed it's not enough to access your funds.
What BIP-85 doesn't replace
Using BIP-85 doesn't remove the need for a backup: now you have a single master seed that concentrates access to all your wallets. If that seed is lost or someone finds it, everything derived from it is at stake.
Source: BIP-85, Motivation