By not setting the ambient key too high ofcourse. Ambient is what it is, it gives an ambient glow to your whole map. This is usually very useful to compensate for the ET engine’s limits.
For example, in the real world, even if you are in your room at night, there will still be light (streetlamps, moon etc) that has bounced like 923858 times from other objects and finally reaches your eyes.
In ET, light bounces like this are also simulated, but only at a maximum of 8 times I believe.
What you get is that in a room, if you are not using ambient lighting, places like corners or the floor underneath tables will often have NO light at all, they will turn completely black, which is unrealistic.
To compensate, you use a little bit of ambient, which will light these places up just enough to seem realistic enough.
In an outdoor + indoor environment (like Fueldump) you use ambient only slightly for the indoor parts and you use lights and skylights for the indoor and outdoor lighting.