Here's the basic idea....
You agree to give up roughly an hour or two of your vacation to take the DVC tour. In return, DVC gives you a couple of instant fastpasses to "make up" for that lost hour of your vacation. (Instead of standing in line at Space Mountain, you sit at the DVC Center).
Ice cream: just a yummy treat as a way of saying thanks. If you're willing to give up two hours of your vacation just so you can get a free single-scoop cup of ice cream, then your priorities are way out of whack. go for the DVC, and enjoy the ice cream as a special treat afterwards.
Other incentives: there are occasionally other incentives for BUYING, not touring. These include occasional "additional down payments" paid by DVC on your lease... for instance, they may take an extra few bucks of each vacation point you buy if you sign that day. These incentives are usually NOT enough to sway you to buy right away, but are a nice thank you if you ARE ready to buy.