If I recall correctly ... all laptops had to be switched on but the amount of time take for each laptop to boot up - and then shut down, caused delays. As the number of pax with laptops increased, so did the queues. Also, it was obvious to anyone that you could make a PC that booted up and looked normal and could still contain malign substances.
Whilst many laptops are now sitting in 'Pause' or 'Hibernate' and will boot more quickly, they can still contain many things. The supposition must be that some very specific thing is being looked for/prevented.
In this article:
BBC News - Heathrow and Manchester issue must-be-charged gadget advice
Heathrow and Manchester airports have told passengers to ensure all electronic devices carried as hand baggage are charged before travel if they are flying to the US.
The move follows a request from the US that "certain overseas airports" implement enhanced security measures.
A new rule states that if a "device doesn't switch on, you won't be allowed to bring it on to the aircraft".
However, it is being applied differently at the two sites.
My emphasis to show the further confusion being extended to the traveller.