The answer might come a little bit late, but (and this is as I understand it), Reparam Steps Per Segment affects the interpolation between any two given points in a spline in Unreal.
When you are working with linear spline points, you don’t really need to worry about this setting as the segment that connects two linear spline points is a straight line.
However, this setting becomes really important when dealing with curved sections in a spline, as it affects the smoothness of the curvature present between two points. A good example where this is important is in the Camera Rig Rail component - if you use that actor to define the path that a camera should follow, and said path contains curves, increasing the Reparam Steps Per Segment in the spline sub component will smooth out the trajectory of any cameras attached to the rail.
Hope that makes sense 