IF your extruding along the curve you might want to know which way its going to extrude..
If your making a motion path using a curve you'll wan to know its direction..
If your making a surface form the curve you'll wnat to know its direction then so your surface dosent end up all over the shop..
Just a few reasons off the top of my head why you would need to know its direction.
"No pressure, no diamonds" Thomas Carlyle