Fertilizer is often applied when field conditions are not optimal, particularly in large scale operations.  Most studies, indicate that nitrogen losses can be reduced in these situations when a urease inhibitor is applied to the fertilizer. 
Urease inhibitors prevent the urease enzyme from breaking down the urea.  This increases the probability that urea will be absorbed into the soil after a rain event rather than volatilized into the atmosphere. This causes subsequent hydrolyzation to occur below the soil surface and decreases atmospheric losses. The use of inhibitors also decreases the localized zones of high pH common with untreated urea.