This is part of the effort to detect visa overstayers. CBP tries to measure this.[1] As of the end of 2015, they estimate that there are 416,500 people in the US who overstayed their visa. They want to use the same identification techniques at entry and exit and match them. This tells them who's entered the US legally but now needs to be found and deported.
[1] https://www.dhs.gov/sites/default/files/publications/FY%2015...