Lightning and nostr are different networks. Yet one address can serve them all. My nostr nip05 is erik@nostrplebs.com and I connected that to my alby address. So now my NIP05 became my lightning address too ;) So, you might not need it, but it can be convenient.

If you have a personal site or a business, it allows you to proof you own it. It's a way of linking your identity to your internet domain.