the algorithms used in Nostr clients and possibly in some relays plus a readable address

the most obvious reason is to have a readable address, you don't go a tell your friends, hey, follow me in Nostr and here is my address and you give them your npub... not very appealing, instead, a readable Nostr Name is easy to remember and store in contacts, like an email, which is cool and convenient.

the not so obvious, Nostr Name increases the odds of being a real user instead of a bot. being an open platform the amount of fake accounts and bots is growing so the algorithms created for content and some clients like Primal will shadow ban from their relays users with no check mark, meaning, Nostr Name

hope that helps