It's actually about sacrifice and slack. If you set a timer 5 mins ahead of the meeting, then the slack/sacrifice is yours. If you are late every time, the slack/sacrifice is his.
So, my explanation is a little different from the many (American ones?) here. It's not that your boss is your boss and you should do unquestioningly whatever the hell he wants. That seems like an awful, butt-kissing environment to work in. Disappointingly many answerers, think that is completely normal, don't even have a concept of how things could be different. (As an aside, look at Trump, this is the exact environment he wants, which is why he's a prime example why corporate jerks should never go into politics, they're used too much to yes-men around them, and not taking responsibility, but blaming underlings).
The others do have a point with the timer, but they're also kind of missing something. They're reasoning from authority. I reason from office efficiency: his time is likely more limited or constrained than yours, so he has a right to demand it not to be wasted.
