One of the worst things that can happen to you when you are traveling is waking up one morning and looking at the clock only to realize that you slept in or did not make it in time for your flight or your bus/train. You may think that phone alarm clocks are good enough, but you would be mistaken, which is why you need a travel alarm clock to keep you on track during your travels.
First and foremost, obviously you need to carefully read your itinerary accurately and know when you must get up for the flight or the place you are going to. Usually, for flights, you should be at the airport 3-4 hours before departure and for buses/trains 1 hour before departure.
Phones are not the most reliable alarm clocks, because the battery can die or the local time zone happen to go through a glitch and your phone does not show the correct time. Or sometimes the alarm application simply does not work. These things can cause a lot of trouble and though very unlikely as they might seem, it does happen.
To prevent yourself from mishaps you should carry a small alarm clock that is set moreso made for reliability and accuracy. Since people are more likely to mess around with their phones, applications and what not, it can jeopardize your wake up time. A simple alarm clock, you just set it and forget it until you wake up.
You should obviously change the alarm clock battery from time to time, but they do last much longer than phone batteries.