[Kylospylon]

I believe if you have your stealthy characters in between the enemy and the rest of your party, they act as a 'stealth-shield', enabling you to get a bit closer to other creatures before they notice you.

I also believe that you are more difficult to spot in darkness, though I have very little evidence to back up this theory. When I play, I do notice a difference in day and night concealment levels, it may be my imagination however.