You need some kind of delimiter between the name and the domain. My guess is we just made it like that cause nip-05s work in a similar way to lightning addresses and that’s how lightning addresses are formatted.
Discussion
The problem nip-05 solves is giving users a human readable unique identifier on nostr. It’s easier to share a nip-05 than an npub.
It can also prove you have access to a domain. For example, if you’re well known and have a personal website and use that for a nip-05, then people will have more confidence that it’s actually you and not someone pretending to be you.