As I use the sketcher tool on a plane or a surface, it snaps the geometrical constraints from origin/other lines/points etc while sketching.
But when sketching on a surface of a solid body (or maybe on a face), It does not snap geometrical constraints from the solid body. I was using this feature in Solidworks which I find it very useful, however, I could not find any options or buttons for that.
For example: I want to sketch a square on a solidbody's square face, and I want one of new square's point coincidence with the corner point of the solid when I start drawing the square by clicking on the corner of the solid.
Select the elements (plane and line, point and point, etc), use the gumb "Constrains defined in dialog box" and there are some of geometrical constraints like Coincidence, Paralelism...
Or select the elements, use the gumb "constrains" then you will become distance or angle, but then click with your right mouse click, and you will becom all possible geometrical constrains for the selected elemens.