Using a Shapes .outMesh Plug and driving an others Shape .inMesh is a really powerfull trick. It gives you the possibility to let Shape A inherit its form from an Shape B, but still beeing able to make changes on ShapeA (unlike, if you would use for example an instance.)
You rig a character, of course you´ll create blendshapes for your characters head. What would you normaly do? You would duplicate the head a couple of times, create your blendshapes and connect them to your head...
BUT, what if you have make some minor changes on your original head (moving some CVs)? you would have to update all your blendshapes, or create an additional blendshape just for the correction, or of course download a script that does the job for you. Anyway, not a nice workflow, and if you use the outMesh - inMesh connections you dont have to, because your blendshape heads would update themself if you make changes on your original head!