You get alot more control in a cgi shot when assembling the final shot from alot of disparate pieces. It is common to render (the process whereby the 3D program takes all the objects, lighting, textures and camera position, size of frame and so on, and effectively draws it as a colour image) parts of the scene separately, according to how much control you want and how complex the shot is. In this case we rendered the fiery torches separately, blacking everything else out, which allowed us to adjust their brightness later and add glow to taste.
Rendering can take many hours, even days, and so it is wise to create a separate basic element you can adjust at the end during compositing, blending all the elements together. Had we rendered it all at once and found it didn't look right, we would have had to render all over again!