It’s a cliché, but the thing about clichés is that they are often founded in truth!
If your PC, tablet or phone was misbehaving and you handed it to someone who knew about such things, it is quite likely they would hold down the power button until it turned off. They would then, most probably, wait about ten or twenty seconds, while whistling, tapping their fingers on the table, or making small talk. Then, when a reasonable amount of time had passed they would hold down the same button and wait to see what happened when it flashed back into life.
Often these simple steps will reinvigorate the device and help it complete whatever task it was trying to process before the reboot.
If the device doesn’t have a power button, as with things like routers which run your home network or land-line telephones, then pulling the plug out, waiting the obligatory ten or twenty seconds, then plugging it back in again will cure a host of evils.
For a very good explanation of why turning it off and on again is necessary go take a look at this article.
IT's Easy 