Green is not the ONLY tailgate lot, but it is the only one that allows people to arrive early and to park wherever they want, not where a parking employee tells you to park.
This allows large groups to set up tailgate areas involving multiple vehicles that do not all have to arrive at the parking lot at the same time, as you can "save" spots in your area. I've been tailgating with a group that parks in one particular row, and then we set up a bunch of tents, cooking equipment, tables, etc. A few people have some fencing and rope that they use to create some space around the grills and tables.
You also get a bunch of people who park elsewhere and come through the Green Lot, some to tailgate and others just to wander around. My group tends to make way too much food and have extra beverages for visitors.