I’ve got a use case related to trust levels and Discourse Meta itself.
In one of my communities, we are using Trust Level 3 as a way to identify those who want to help improve our product and community, which includes our own usage of Discourse. It’s like our own version of Discourse Meta. Naturally, I find myself frequently quoting conversations that I find here.
That is, by discussing Discourse Meta’s topics about the Discourse software, my community is having conversations about conversations about conversations. We are all indeed a strange loop. 
Anyway, I see value in inviting the smart people from my community to meet the smart people in this community — all of you fine people reading this. However, because T3 is dynamic, email invites won’t work. Although I could simply invite all of my users with a link here to /signup, it’s only the T3 people who have really proven to have interest in community introspection and curiosity about the software we use. Plus, I feel that having my T3 users discover such an invite as a reward would have a stronger impact and motivate them to really get involved with the larger Discourse world. After all, by helping Discourse, we are also helping ourselves.
If I had an email agnostic link that all my T3 peeps would see the first time they visit our Lounge, I’d have a practical way to bring more people here and also get myself that Champion badge on Discourse Meta. 
I suspect I do not have the only Discourse community that could do such a thing. Also, while this specific example would benefit Discourse Meta, obviously I would also want to use it within my own community so that my own members could more easily become Champions themselves.