Right now to add a new component to the layout from the component menu you just click on the thing you want to add. You have very little control over where it gets added to the layout. If you have a component selected on the layout, and it has open connections, then the new component will be automatically connected and positioned accordingly. Otherwise it is placed towards the upper right corner of your screen.

Drag/Drop will allow a user to drag (using their mouse) a component from the component menu directly to where they want it on the layout. Clicking to have it automatically added will still work, but this will give you more choices and for some people will greatly speed up their workflow.

Note: This is only an improvement for non-touch users. Since touch drag is used to scroll through the list of components/parts, it can't be used to dragging a part onto the layout. Maybe in the future some UX person can suggest a way that it would be possible though.