I can look at a histogram of a camera I know, and get a pretty good idea of what I will have to work with when I get to Lightroom or Photoshop. It's like working in a professional kitchen, yes, you can use tools to get somethings exact, but when you are in the 7pm rush, you don't always have time, and you just learn your tools and instincts. When shooting events, it is the exact same thing. I can glance at the histogram and in a short time know if my exposure is going to give me workable results. It isn't about being exact, it's about getting a point that is workable.
I'll break it down on how I use the histogram.
1. Are white and black all the way in. If they are, then great, if not, how badly are they out.
2. How do the different "zones" of the histogram look? If I have a lot of room to the top, I can shift exposure to over expose knowing this will reduce noise in the shadows.
3. I hate clipped highlights on digital (the roll off is too harsh, while film feels much better, at least to me), so if I see way too much data on the high end of the scale, and lots of extra room at the bottom (I don't mind clipped shadows, and almost always process towards an "inky" black), I'll shift exposure down.
In about a second, I can do those three steps in my head. If I want precision, I'll meter with an external meter.
*edited: removed a slightly inflammatory intro. I mean no ill will, and value DigitalDog's different perspective.*