Lump Jockey,
The dew point is important to know for a few reasons
1. A big dew point/temp spread can be a good indication of future CB's etc ie unstable weather
2. A narrow dew point/temp spread can give an indication of possible saturation and therefore fog/mist etc
3. A high dew point can tell you of a high density altitude which will decrease aircraft performance or increase the likely hood of carb ice if you are flying a piston aircraft due to a high moisture content.
Hope this helps