It's not always about making an immediate impact. A subdued voice can have subtle, deeper effects on listeners, and it's the ideal voice type if you're looking to bring a hushed, mysterious quality to your projects.
Some voices explode, and others implode. The mumbly, inward quality of a subdued voice is for the latter type of character, the ones who speak in hushed tones that usually barely rise above a whisper. This voice type works great for downcast, diffident, or nerdy characters, as well as confident, mysterious types that never seem to show their hand until later in the story. Whether a character's an introverted loner or if you need them to have a giant question mark next to them every type they appear, a subdued voice is a great choice.
Not every character in media is a heroic, boisterous type. Some lack confidence, or are simply quieter, more introverted types that do better in the lab than on the battlefield. Other characters are mystifying, peculiar and inscrutable, and you wonder what's up with them for nearly the duration of the story, as they seem to have an impenetrable, unflappable aura. The subdued voice is great at both creating questions around a character, or generating empathy with their sheepish, withdrawn nature. This voice type also works very well in marketing, both to create characters that break out of their shell, or to generate humor or ironic counterpoints.
Low-volume, hushed, and with a generally despondent, introverted quality that doesn't lend itself to a rushed diction or high-pitched tones.