It's useful if you need to prove ownership of a domain. If you have followers, customers, friends, ect outside of nostr it can help them find you without you having to invite them with your nub. 
It provides a human readable way for people to ensure you're not an impersonator.
It's not currently being used to prevent spam but may in the future.

And Imo it's only relevant in a conversation about verifying your own domain. Using a nip-5 service serves no benefit unless other extras are included (like a private relay or media hosting).