Adding negative keywords to your AdWords / Bing campaigns can help you get higher quality clicks for your spend. If you look at Dimensions in AdWords you can see exactly which search terms landed you clicks and conversions. Finding that a keyword like "B2B marketing plan template" lands you a lot of clicks but no conversions, for example, might be a strong sign that you should add that keyword as a negative so your ads don't appear on that keyword. This can save you a ton of budget for higher quality clicks.