Another key point is that [acronym=Quick Develop]QD[/acronym] applies relative adjustments to multiply selected images, while the Develop module applies absolute adjustments. The best way to illustrate this is an example.
Patrick on another thread was interested in adding 2/3 of stop exposure to a collection of images. Many of the images had already had tone adjustments applied, including exposure. In the Develop module, any adjustments he applied to multiple images via synchronization, or copy/paste settings ,would have set all the selected images exactly equal to the numeric result of the 2/3 stop increase on the active (most-selected) image. This would have effectively overwritten the previous individual adjustments. Result, each image has the same exposure setting, regardless of its previous setting.
In QD, Patrick can select all the target images, click the '>' exposure icon twice, (each click is 1/3 stop), to add 2/3 of a stop to the 'current setting' of each image. Result, all images are brightened by 2/3 of a stop, on top of any existing adjustments.