The free Quick Attach addon for blender is essentially a streamlined way to apply a “Transfer Weights” operation.
“Transfer Weights” allows you to take the vertex weights of an object that is already skinned to an Armature, and apply the same weights to another unskinned object. In effect, both objects will now be deformed by the character’s armature in perfect synch.
One of the most common use-cases for “Transfer Weights” is attaching a piece of clothing (such as a jacket) to the character’s body.
While you can perform this operation without an addon, that involves several steps and tweaking settings, which may be difficult for less experience users. It is also time-consuming.
Quick Attach simplifies the entire process to a single click.