Within the game audio industry, ambiance falls into two distinct categories: Sounds the environment produces and harmonious elements associated with the surroundings, such as tones, swells and transient sounds; all of which are meant to evoke emotion from that location.
Sounds the environment produces can be manifested in many different forms. For instance, in a forest you might hear the wind, leaves rustling, birds chirping, grass swaying, etc. These can be directly associated with real life sounds. They can be fine tuned and exaggerated, or dumb down. This all depends on the creative vision.
The second category of ambiance takes precedence over the latter and dictates what you "should" feel. Your safe isolated forest can quickly turn into a danger zone, by adding one eerie wind layer.
By combining the two, you reinforce the direction of the playfield, be it unearthly, dark, possessed, suspenseful, or whether you want to play it peaceful and safe. These emotions can be achieved without music and the advantage is two-fold. This allows the subconscious to decode the information without music constantly pointing at the obvious. Deception can be used as a tool, playing off of anticipation. If 95% of the time your peaceful ambiance represents a safe zone, 5% can be used to fool the player into an attack. This introduces a dynamic environment, an additional sonic cue that can be more powerful than music as it is more subtle and the player becomes conditioned.
One of the biggest advantages is that players of certain genres are known to turn the game music off. This is understandable if you're an mmo player that cranks 25+ hours a week. In this event, the sound effects and dialogue remain intact and your dynamic environment becomes just as important. We all know how crucial it is for competitive players to receive audio cues!
These are decision that need to be established early on in pre-production, by designing consistent sound palettes and implementation techniques. What do your environments need to sound like? Will they be more realistic, magical, or maybe a combination of the two? Is there a day / night cycle? How is your engine going to play these sounds back? Will it be stereo, quad, surround?
A bit of planning will ultimately benefit the player significantly.