The Nostr repo describes the protocol as "The simplest open protocol that is able to create a censorship-resistant global "social" network once and for all." Making the exchange of human-readable text (a core part of being "social") a mandatory part of the protocol makes perfect sense with how Nostr describes itself.

https://github.com/nostr-protocol/nostr?tab=readme-ov-file#nostr---notes-and-other-stuff-transmitted-by-relays