The free issues are specifically produced for this event in the hopes of hooking young (an old) readers on comics. It's usually one or two-per person on the free rack, but if you're going with kids and everyone gets something different you can all share in the spoils. My girls loved doing this last year and discovered Owly as a freebie. It wasn't far before we drifted to the racks where the Mickey Mouse comics were and those were a continued hit over the summer. This year there's a free Mickey in the bunch, and an Archie, and a Spidey so it looks like goodies all around.
This would be a good time for those interested in graphic novels to visit and see what's out there. There's a pretty big pond out there if you've never taken the dip; most regular book retailers don't even carry a fraction of what's available including a large number of indie comics publishers who can't swing major distribution deals. It might be too busy to chat with the retailers on Saturday about recommendations (though I've yet to meet a comic retailer who didn't love to talk shop) but if not ask what they like for kids and get a whole different perspective on what kids are checking out in a different section of the kidlit marketplace.
Once more, with feeling: Free stuff.