The human voice has a very wide variation in volume, called the dynamic range. Think of the difference between the quietest whisper that can be heard in a room and then the loudest booming voice.
This range is too much for most recording equipment. If the sounds coming in are louder than the limits set on the equipment, the recording will sound like a Dalek convention.
At the risk of over-simplifying, it might help to think of the dynamic range as a very wide goal, with the recording equipment settings as the goalkeeper and the artist playing a back-pass. A poorly skilled artist could potentially kick the ball anywhere toward the goal and the goalkeeper can only cover a small fraction of the goal. If the ball misses the keeper, it’s an own goal; the recording fails.
Sound Engineers must calibrate their settings (place the Keeper) to the expected Voiceover Artist’s delivery and the artist is responsible for helping with this. Once set, the artist must deliver to stay within those settings.
The artist must be acutely aware of their volume and other things that affect the recording, such as the distance to the microphone and the point on the microphone the vocals deliver to.