Sneaking by holding the left-shift causes your character to be less detectable. How does this differ from moving or standing normally? Under what circumstances does sneaking prevent you from being detected wherein you'd be detected if you weren't sneaking?
Sneaking is only useful when moving. While you are standing still it does nothing.
Sneaking is more specifically useful for the cleaner because he can sneak up on enemies without them noticing he is coming.
When you are trying to get around an enemy, it is better to sneak rather than run since enemies can't hear you coming. The question mark (detection meter) above enemies' heads fills up faster per distance unit when not sneaking
If you are going for a speed run, sneaking might not always be the best option since it does take up time.