While it’s not clear in the graphical display, curves do have a direction and a start point. You might want to reverse a curve
Changing the direction of a curve has a number of possible uses:
to change the direction of a sweep in a swept surface
to change the direction of a curve in a surface
to change the direction of a curve as you are joining curves together
Changing the start point of a curve is helpful for operations such as Ruled surface. The start points of the two curves are used to form a correspondence between the curves when the surface is created.
How to reverse a curve's direction
How to change the start point of a curve