@zvava@twtxt.net @bender@twtxt.net At first I added it without thinking when planning the possible fields based on other UIs I was researching.
I was about to discard it but after thinking about it a bit I noticed that the services allowing to have a separated nick and display_name could unlock some good uses.
For example some added context or at-a-glance information like pronouns or statuses (like Artist [Accepting commissions] or App Name (v2.5)) while other used a more readable version of the nick (blog.domain.com became Person Name's Blog).
Of course it is absolutely optional and it can be safely ignored, but with my vision of being able to build more that a pure twtxt clients, giving it a first-class support just like the other known fields felt right to me.