There are plenty of reasons to make a shell corporation. For instance, suppose company X and company Y are working on a joint venture. It makes a lot of sense to make a shell corporation, fully owned by X and Y, to actually do the work of developing the joint venture, owning any patents for it, and licensing or marketing it.
This keeps things nice and clean for X and y, and if one or both ever want to get out they can easily sell their interest in the joint venture by simply selling their shares of the shell.
Basically, doing something in a separate corporation you own instead of doing it directly under your main corporation turns it into a single thing (your shares of the separate corporation) instead of a bunch of things.
It's essentially the business organization equivalent of a library or module.
This keeps things nice and clean for X and y, and if one or both ever want to get out they can easily sell their interest in the joint venture by simply selling their shares of the shell.
Basically, doing something in a separate corporation you own instead of doing it directly under your main corporation turns it into a single thing (your shares of the separate corporation) instead of a bunch of things.
It's essentially the business organization equivalent of a library or module.