When we turn things on or off, there is surge of electricity through the components. If a component is starting to fail, this surge may be enough to make them fail. It used to be a big problem when things used valves but is less of a problem with modern circuitry.
In standby mode, the devices still use a little electricity and there are people who would say we need to cut ALL power usage, no matter how little difference it makes in order to reduce the pressure on the power networks and the generation industry.
That is a silly argument when the same people say we should be driving electric cars that require MASSIVE amounts of electricity or who use contactless phone charging which is INCREDIBLY wasteful and inefficient.
Overall, I don’t think there is a convincing argument for leaving on standby or switching off. I choose to switch off and, since my first computers back in the 1980s, this has not caused any problems.