Faders (or "directional", as you call them), are normally assigned to track volume and can thus be used as direct mixing control. Rotary knobs can be assigned to any track or effect control in your DAW. For example in my controller the default assignement is track pan.
But both types of knobs can be assigned to different controls in your DAW (depending on the level of customization supported by your DAW and the controller). Normally you would choose rotary knobs for controls that look like rotary knobs in your DAW, and faders for controls that look like linear controls in the DAW, but it's up to you.
Depending on the level of software integration of the keyboard/controller with the DAW you use, some assignments may be done automatically, e.g., if you select an effect that is placed in a track, the rotary knob in the controller immediatly gets assigned to the main parameter of the effect. If you deselect the effect and go back to the track, it get's reassigned to pan (for example).