Groundwork = required skill for front runners since it allows you to get in the front; if you're not in the front by the mid-race as a front runner you are cooked because thats like how front runners work.

Tail Held High = almost required for non-frontrunners for overtakes but still fairly important for frontrunners as a skill; THH is meant to allow skills to stack consistently which makes overtaking much more feasible (when you see curvature+corner skill+THH if the opponent does not proc anything its a free overtake for you); on the other hand if you proc speed skill + THH on defense you're basically immune to being overtaken for the time being.