The answers in SimpleForm without for (non model form) indicate that it's possible to create a form using SimpleForm without a model being used.
However, I don't know whether it makes sense to do so.
What benefits does SimpleForm provide over the forms generated by Rails when you don't have a model object involved?
The benefit that it provides is that you don't have to build any html around your fields, and add classes to them, if you properly configured simple_form. Because for example for bootstrap you mostly will have some wrappers around your inputs.
And you also have automatic error messages in your form this way.