The 3D Sketch environment was added several releases ago, and in ST10 it is being enhanced to help you work in some of the environments that require 3D wireframe type data. Specifically, PathXpres is now available in 3D sketch under the name Routing Path.This helps you get point-to-point straight line segment 3D sketches quickly.
Another 3D Sketch enhancement in ST10 is that the cursor for line endpoints automatically snaps to the center of a circle or arc when you hover over an edge. This makes centerline sketching much easier.
If you want to snap to another keypoint such as the endpoint of an arc instead of the center, use the Shift key.
Also, when you drag that line out from the center point, you get a Coaxial relation, as shown in the screen capture.
All of this is obviously intended to make tube/pipe layout easier.