For me, a few reasons. One, when I wake up in the middle of the night, I like to see what time it is. I don't want to have to fiddle with a phone to get the time. I know can make my phone's clock stay on the screen, but then I have to either pick it up and look at it or bring a charging stand. Also, I have my phone set to go intot total no sound, no light, and no vibrate mode at night. I don't need to be woken at 4am by an alert that Amazon recommends something to go with the crap I bought last week or that my car warranty has expired..
Instead of thos hassles, I travel with a small LED alarm clock that projects the time on the ceiling. All I have to do is open one eye and look up to see what time it is. It's USB powered and also has an extra USB charging port. To me, it's much more convenient that using the phone. Having said that, I rarely ever set an alarm. I usually wake up around 5-6am naturally, so unless I need to get up earlier than that for some reason, no alarm is necessary.