>>126637Avoiding the Redshirt concept (point 1) and screentime (point 2).
The Redshirt concept is that the characters are disposable and are there just to die. Not to say you can't have them die, but give their purpose something more before dying. Their actions and subsequent deaths push the plot along, maybe they sacrificed themselves to buy the heros time for example. Or for a more in depth example: a group of knights storm the walls while the main characters are sneaking in through tunnels. Many of them die but they succeed and the result has a knock on effect that aids the main characters, then at the end we see what remains of the mobs they had a brief interaction with earlier. While they were never the focus and we only see the before and after, it still gives us a glimpse of a wider world even if we were focused on the main characters.
Mobs are either dressing for a scene or at best they're minor characters that show up once or twice that interact with the main characters, but that's not to say they're unimportant. Their job is to do things the main characters cant do and support/hinder them in the grand scheme of things. The main characters cant exist in a vacuum and thus need mobs to hold up their share of the story, the background cast and extras exist to give contrast to the main characters and give the world some semblance of life and potential stakes to care about.
As for importance, I assume you mean to the story. The actual power of the main characters and mobs are irrelevant, what matters is how their interactions and screentime affect the story told around the main characters.