Presenting things as maps is the 21st century method of establishing credibility. It is much cheaper than actually creating an informative graphic with useful data, a la Edward Tufte.
As has been mentioned, this data is not all that useful. One still pretty much makes more money in places that are more expensive to live, but not nearly enough. For instance, in San Jose one might make 25% more than in Houston. On the other hand, the median price for a home in San Jose is 5 times the average pay listed on the site, while in Houston the median price for a home is only 1.4 times the average pay listed on the site. Doing this type of normalization is simple, but does not drive rubes to higher paying jobs that might pay higher commissions to places like Dice.