Preseason games are often a detailed, sometimes depressing or desperate search for depth. That is especially true in Week 1 when the starters' time on the field seems shorter than your average commercial break.
Teams want to know where they have depth so when the time arises the next man up can be trusted. They also want to know where depth is lacking so resources—both the coaching and development kind, and the player acquisition kind—can be allocated appropriately.
Early answers, or at least strong hints at answers, come quickly when second- and third-team players get extended playing time.