Illustrator allows to set a key object in a selection with a click. The object gets thick edge and can be used for aligning and distributing operation, to stay in place.

This simple way of marking an object is used only for these, and it’s a waste.
We need to be able to use key objects for other operations as well.

For example, why not allow users to clip into the key object, if it’s set? Currently we would have to move it above other objects in a selection, reselect them together, and hope we didn’t mess it up... or cut the objects we need to clip, select the one to be a mask, toggle Draw Inside mode, Paste in Place (works if we need to save the appearance of the clipping path).

Why just not
1. Select objects
2. Click one to make it a key
3. Run Object > Clipping Mask > Make command (Cmd/Ctrl+7)?
Zero extra steps. No problems with 'should it be at top or bottom'.