When the user wants to be able to create features in an editable layer quickly that might fall along an existing line or polygon feature in another layer. For instance, scoping where road safety treatments might be beneficial – these may need to follow the shape of the road and may only be parts of a road without having to snap node by node. There may be a similar need for polygons where it may be quicker and easier for the user to trace around a group of polygons (e.g. land parcels). The added benefit of tracing would be the length or area of the resulting edit would better align. So if needing to know what the length or area is e.g. potentially for scoping up an initial estimated cost per unit length or area then this will make it easier. Having a trace option would really help out and avoid having to go through an offline process of having to locate crudely edited features along a road network then linear reference them based on where they might align then bring that data back into the web layer. If there are constant changes to the treatment extent then the back and forth process between the user's edits online and correcting them offline isn't as efficient as the process could be. A trace option would be a bit of a godsend.
To take it to another level, if you could trace with an offset (user chooses a distance to offset by, maybe have units set in background config to keep as a constant) that would be super useful when the user wants to draw side barriers (left/right) along the roadside which are parallel to the road centreline.
