The problem that this feature will solve:
3D spline function in sketchings
Brief description of the outcomes that you expect from this feature:
For example sketch a spline on a plane that connects to a 3D body. Move the control point out of the plane in 3d space and manipulate the spline. 3D spline can be use as a guideline for lofts or sweeps
What can’t you achieve without this feature?
Most organic shapes that cant created with different planes.
Been asked for repeatedly, really hope it’s implemented though. Planning for wiring, tubing, any other number of things that rarely in real life can be divided easily amongst 3 planes is of enormous benefit. I don’t want to create 50 planes to sketch out the way a wire needs to get routed.
There was another post about this where another user showed how to get the same idea, but it still required multiple sketch planes all in 2d pieced together.
I think shapr works very hard on the parametric version right now. No dev answerd on topics like 3D spline capability. Maybe they worked on it in some backrooms. Hope we got the capability for variable fillets too in the next few months. Would be useful in eyeware design 
Apparently variable fillets are coming sooner rather than later. There was a post awhile back where it will be integrated into the history edit if my memory serves me correctly. Think it would be easier to select points on an edge to define but baby steps.
As for 3d splines it’s been a few years. I remember reading a convo here awhile back where the people requesting it were asked repeatedly “why do you need this?” so I don’t have much hope for that anytime soon.
