What’s the problem?
Most Ruby on Rails applications start off as simple web applications with simple requirements. As a developer, you take those requirements, map out the business domain and then implement it using the MVC pattern of models, views and controllers.
Over time, more requirements are added and simple controllers can become bloated with complex logic. The obvious next step is to create public methods on your models and move the logic into them, thus keeping the controllers simple. However, this inevitably leads to fat models.
One of the guiding principles of Software Design is the Single Responsibility Principle and fat models have too many responsibilities. In fact, you could argue an empty model that extends ActiveRecord::Base already has multiple responsibilities: persistence and data validation/manipulation. Furthermore, an empty ActiveRecord model contains 214 methods (not counting any of the methods on Object). So these classes are already tightly coupled to the Rails framework and if you put your logic in them too, they’re also coupled to the business domain.