Because of the wallet they have to do that, that's what Miljan said.
Discussion
"Current" Nostr Client has a wallet and never needed email. 🤔
I'm not sure, that's just what Miljan said.
That is in case you want to get a primal's LNURL address, and use their wallet. you can use your own if you want.
Still you can use any info there(disposable email etc). I use duckduckgo email protection, works great 😃
They have choice like damus with the post installation script, but when you want to centralize your client you dont have option..