Solar Models allow you to centralize domain logic. Doing so allows you to avoid repetition and to ensure that a consistent set of rules are applied to your domain objects. When developing Model View Controller Applications, its a good idea to follow a principle known as Fat Models, Skinny controllers. That means you should attempt to place as much of your domain logic in your model and avoid domain logic in your views and controllers. Solar Models support this principle.
In Solar, each Model is tied to a Sql table.