Gas fire pits typically come with a "Medium" of either lava rock or fire glass through which the flame is passed from the burner.
The primary purpose of the flame medium is to absorb heat and distribute it more evenly as it's released, thus preventing the heat from your fire from dissipating too quickly.
At the same time, the medium will also change the appearance of the fire in your fire pit. Remember that you can buy different mediums like these Glass Medium Choices to put in your fire pit. This allows you to change the visual appearance anytime you see fit.