I always guessed that they wanted something around 10 minutes, but they wanted more than one visible digit to change each time to make it more obvious that time

I always guessed that they wanted something around 10 minutes, but they wanted more than one visible digit to change each time to make it more obvious that time had passed. (That is, a sleepy person might be more likely to mistake "7:20" for "7:00" than they would be if they saw "7:18".) But that wouldn't be relevant with old analog alarm clocks, which evidently had a 9-10 minute snooze, too.