The use case is a bit more widely applicable, I’d say: I often remember who wrote a paper but not the title. In our field (planetary science), the person who writes the paper becomes first author, simple as that, but shouldn’t this be true for most other fields?
Now, that author might also be co-author on many other papers. Even if this were only the case for a set of maximal 3 authors, a search that does not support “FirstAuthor” filtering gives me many more results than a search that filters out all results where the author’s name is not on the first place.
The fact that this is broken for many years in the Papers app despite several requests for fixing it drove me to the search for alternatives (among many other probs), and I’m a bit bummed that this doesn’t work on paperpile yet. 