If you use multiple clients, but you don't want to import your main private key into some of them (e.g. bacause of security concerns of web-based clients), you can sign a 'delegation' from your main keypair to another keypair used in the secondary client. Delegation can have a reduced scope (time, event types).

Spec exists (NIP-26), but clients don't really support it yet.

I think it will have more need for it when the client ecosystem grows and matures, and there's gonna be some special-purpose clients which can do some actions on your behalf, but you don't want to give them your 'master' secret key.