Sometimes we get into situations where there is so little light that the auto focus system struggles to find the focus. The details in the subject are so dark and so muted that the camera has no way to discern lines that it needs to bring things into focus. Obviously one way to fix this is the auto focus assist light on the camera. But this has some challenges. First, the light is fairly bright and obnoxious. People will tend to look away when the light goes on. Second, the light is fairly dim too. Yeah I know I just said it is bright. But as far as being bright enough to really light up the subject to get a good auto focus the light is quite dim. This is especially true if the subject is any distance away at all. Third, when the light comes on it gets more difficult to get those more candid style shots because people know you are getting ready to snap a picture. This is not so bad for posed shots, but if you are doing some sort of event shoot then it can be a killer.
There is a good solution for this though. A number of strobes have an autofocus assist light setup on the strobe that works much better. The Yongnuo TTL strobes all have this. The Nikon speedlights have this too. The nice thing about this solution is that they shoot out a red laser pattern. First, the light is less obtrusive. Second, the light has a pattern to it that makes for very easy focusing. Third, you can get those candids because people will most likely not even be aware of it. Fourth, the light travels a longer distance than the annoying light on the camera body.
The only challenge here is that the strobe will fire when you take the photograph. If you don't want the light to affect the photo, like maybe getting a sillouette or something, then you can simply put the speedlight on the lowest power and point it straight up or behind you. You could even mask the strobe with some gaffer tape.
Take note that on the Yongnuo strobes it is only the TTL strobes that have the auto focus assist light. For other brands make sure you do your research before purchasing. One final note.... this is nice too because you can now leave that annoying autofocus assist light on the camera body turned off and next time you are at your kids school play or dance recital you won't be annoying the people in front of you when it goes off and is doing nothing because you are like 50 feet away from the stage. :-)