Water controlled by electricity? Are you thinking about hydro-electric supplies there?
This kind of advice is normal in places like NZ where everyone lives at risk of earthquakes. Not sure how many comply.
I guess the German Government must have done a risk assessment on different types of terror attacks which could include damage to transport system (preventing food deliveries), a dirty bomb, chemical/biological/explosive attack on a vulnerable facility. If people can manage at home for a few days it would tend to keep them off the streets and out of harms way, and discourage looting. Remember what happened in New Orleans when it got flooded? Something that disrupts the road network, such as the diesel delivery drivers strike we had a few years back can very quickly cause a major problem with supply of food to supermarkets.
Water companies, and similar, have to make a lot of contingency plans for civil emergencies. I remember once, when working for one, having an interesting conversation with the disaster planning manager. Apparently we had control rooms that were protected from nuclear fallout and huge subterranean stores of generators etc all ready to be used if ever needed. At that time nuclear attack was the thing in the forefront of people's minds. I saw one of the bunkers once, it was in a water control room where they had an inner set of doors that were metal and about a foot thick and the inner room was built into a hillside. There were a few senior managers and essential personnel who knew they had to go there if there was a nuclear alert - and close the doors behind them. From there they could control the water network that fed a major city etc.
what exactly are they expecting. I'll chuck a few tins of baked beans in the shopping basket, just in case there's something we are not being told 
