We actually ask our clients to keep all appliances that need to be worked on still plugged in. Many appliances have codes that are stored in them to help appliance repair professionals diagnose problems. If you unplug a device for a long enough time, it may erase those stored codes, which can make fixing the problem much more difficult.
We also often find that after an appliance is unplugged for a while, when we show up and plug it back in, it’ll work just fine… for a few days. Then, inevitably – since the problem was never fixed – the appliance will break down again, resulting in another service call.