Much like production stills, Unit Stills aim to capture the growth and development of film productions. But unlike production stills, in my opinion, they aren't taken to be used explicitly for advertising material. They exist to show the progression of the production behind-the-scenes, and to try and blur the lines of character and actor.
As such, I believe that Unit Stills occupy two purposes:
1. Capturing behind-the-scenes moments and documenting a shoot from start to finish, hopefully.
2. Find moments within an actors performance that exemplifies their character. There are moments in rehearsal or at the very end of a sequence where a photographer has a fleeting moment to show the actor truely becoming their character.
Some of my utterly favourite works focus on the latter side of Unit Stills, glimpsing a small moment where the still is not of an actor working, but a character telling a story. My favourite photographers who strive for this work are David Darcy or Ben King. If you google any of their work, on the dressmaker or Red Dog, that's the level of quality and drama I want to bring to my work, two truly inspiring photographers to me.