In movies, why do bombs always come equipped with a visible timer?

[Red Squirrel]

If I made a bomb I would make sure the timer is 5 seconds ahead of the actual explosion. I'm evil like that.

5...4...3...2...1...0

*cuts wire just in time*

"It did not blow up, we are saved!"

beeeeep *boom*

Of course in real life, I would just cut the wire going to the detonator, skip the middle man and go straight to the source.

 

[Jeff7]

If I made a bomb I would make sure the timer is 5 seconds ahead of the actual explosion. I'm evil like that.

5...4...3...2...1...0

*cuts wire just in time*

"It did not blow up, we are saved!"

beeeeep *boom*

Of course in real life, I would just cut the wire going to the detonator, skip the middle man and go straight to the source.

Feed the actual number of seconds remaining through a hashing algorithm, then display the result.



Or...

"When this random number generator reaches zero, you'll be executed!"




Bomb threat in high school: After that happened, our physics teacher spent half of a class talking about how stupid the whole thing was, and started talking about bomb designs that would be "resistant" to detection. (Like put one in a drop ceiling, and make it sensitive to the motion of someone lifting the ceiling tile, or to the IR of body heat.)

It was a small rural school. There were hardly any fights anywhere. One act of "vandalism" I remember was when someone inflated a rubber glove and taped it to the top of the trophy case.
Our homemade bombs would have included an apology note in with their tissue paper confetti shrapnel.

 

[_Rick_]

I'd include a visible timer for debugging purposes.
It's convenient, as a bomber, to know that the timer has been correctly set, and is counting down in the correct manner.
It also helps prevent inadvertently setting off the bomb.
When you're building your own, that's an important thing to consider.
Also, using a classic alarm-clock type timer allows you to use the beeper impulse to trigger the bomb. That's quite convenient, if you're restricted to using home-depot grade engineering.

While some artistic license is taken, and a modern bomber would use a remote controlled detonator, or a combined timer/remote controlled device, most likely based off of mobile phones, the visible timer meme holds up to superficial inquiry.

Taking the battery out is probably the most effective way to defuse an electronically triggered bomb, by the way
Especially since it's likely that inserting the battery is how the bomb is primed in the first case. Unless there's a capacitor-based booby trap placed on the battery after the bomber checks that the electronics check out, and decides that the point of no return is reached. But that's dangerous in itself, since it increases the risk of premature detonation, and is tricky to do under field conditions. It surely requires extra practice for the operators.