Do I need to have a control surface to use Soft Arranger?

No, you don’t need a control surface. Everything that can be done from a control surface can also be done from the computer screen using your mouse. 

Obviously if you are using Soft Arranger live in front of an audience you probably won’t want to use a mouse because it just looks unprofessional and its use requires a level of precision that you usually don’t have when performing on stage. 

Lastly keep in mind that you can’t compare the feeling of freedom and control that you have when using a control surface.

This is why even though it isn’t required, it is highly recommended to use a control surface for a full Soft Arranger user experience.