Having a NIP-05 address opens a lot of doors for single-sign-on, beginner friendly UX, bridges to other systems etc.

It works both ways.  Systems with a NIP-05 type identifier (e.g. that I'm building into solid-lite) can then also use nostr on top of their current system. 

 What are examples of non-nostr systems with NIP-05 ID's?  

 It's more user@host type IDs than nip-05 itself.  NIP-05 just gives you back machine readable data from that common pattern. 

 So what you mean is that now any app that let's users log in with "email" can integrate nostr on top of those log ins without necessarily custodying their users nsecs? 

Really trying to see why and how this works in two ways.