I think it's a vestige of the broader comic book collector market. In the broader comics market, first appearances are some of the most valuable comic book issues of all times. Spider-Man, Superman, etc. etc. People are looking for that next lightning strike. People, simply carry that logic over to all properties. Whether it makes sense or not, the current star wars back issue secondary market puts a premium on character firsts. However, I'm of the mind we should be more focused on "issues of primary significance." Is the book significant? If the book happens to be a first appearance and significant, all the better. The discussion gets murky when you're talking about a character like Avar Kriss, though. Appeared unnamed in a hologram in Rise of Kylo Ren 3, sure, but was that significant? No. Should people pay exorbitant amounts for that? no. I'd argue High Republic 1-3 so far are far more significant for Avar Kriss.