Do I need a Controller?

No,

However, it is very useful. 

You could actually get a stripped down setup such as receiver, cables, any motors you need and control all via Thumbwheel/s or Smartknob. Often a good option for solo operators. 

The Controller is used for wireless RF control of motors as well as changing any settings of the system such as motor class, wireless power, wireless channel. In addition you would have more precise axis control with dedicated knob, slider and dials.