This adds an additional way to scrub the playhead, by holding Alt and dragging with the left mouse button, in addition to Shift-RMB.
It's easier to do this, especially with pen input. The other method is still kept, in case you have Emulate 3 Button Mouse enabled, in which case Alt-LMB pans the view
And of course, you can still scrub simply by dragging the playhead, without holding any modifier keys.
Right click select is unaffected.