Ice cream is an igneous rock made up of ice, air, and sugar.
Much like igneous rocks, the same liquid mix can turn out very differently depending on what happens while it is freezing. The goal of most ice cream and sorbet is to have a smooth and creamy texture, which would be ruined by the presence of large ice crystals. To achieve this, you want to cool your ice cream so quickly that the crystals don't have time to grow, and keep the mixture stirred up while it freezes. There's a lot of energy involved in the transition from liquid to solid water, and a home ice cream maker can't do the heat transfer quickly enough to keep the ice crystals small, so you have to sit there and turn the crank until your arm is sore while the mixture slowly freezes (or invest in a fancier machine that will do the stirring for you).
See also what happens when ice cream sits for too long in the freezer and the book, The Science of Ice Cream.
