Bingo!! CamoCop nailed it. These public land areas, be it state or federal have to have funding in order to operate. In order to get that funding, they have to justify the need for it. They use the sign in/out process to justify the funding needs based on the number of people who are using the property.
Also as Cop mentioned, it's to ensure that everyone got out. If your family knows you'll be hunting that land and for some reason you fail to show up back home after hunting and she calls authorities, they can check the log. If you never signed back out...maybe there is a problem. If you did sign out...maybe there is still a problem (your at the bar [8D]), but not a problem they need to worry about.
ORIGINAL: CamoCop
alot of our management area's have the samed rule down here. it's not a dumb rule either. the rule is in place so the state can keep track of the lands useage for statistical data AND to make sure everyone that is hunting each day makes it out safely.