I would assume they help keep track of / index the massive amount of government-sponsored agricultural research & data the DoA has generated. "[The Agricultural Research Service] has more than 2,200 permanent scientists working on approximately 1,100 research projects at more than 100 locations", to say nothing of historical data.
http://en.wikipedia.org/wiki/Agricultural_Research_Service
http://en.wikipedia.org/wiki/National_Agricultural_Library