This button would stop everything the machine does. It would not draw energy anymore, it would not assemble any more things, it would not accept any new resources from inserters / pipes. It would not delete already begun, but just pause the progress bar. Un-stopping the machine would let the progess bar continue where it stopped. A small symbol should be showed then, maybe some "zzz" for sleeping.

Why should we need this? Every other possible way of deactivating a machine has some drawbacks. Removing the machine or removing the order deletes things in progress and leads to forgetting which order was intended for this machine. Removing pipes or belts could distrupt other machines. Removing the energy is problematic because mostly other things are also stopped, but then again only removing the energy connection has the advantage of not drawing any stand by energy.