Some years ago, I recall there was a bug in iOS which caused alarms not to update correctly when DST started (or ended), resulting in lots of oversleeping as alarms failed to alarm when expected.[0]

Personally, I have a problem with my cheapish Android phone where it will suddenly and for no apparent reason decide it's been moved to UTC, and move the time ahead to UTC. It forgets about this delusion when I restart it. So far this has only happened in the middle of the day (and all it's done is mess up the period it's supposed to be silent during, when I'm at work, thus the downside so far has been a couple annoying dinging noises at my desk).

Also, I have a fancy alarm clock which alarms by gradually lighting up the room, instead of by blaring loud noises. I find this a more agreeable way to wake up in the mornings. I also like having a dim, but legible time display on my nightstand to glance at when I wake up in the night, rather than having to press a button on my phone, which is then way too bright to look at in my dark bedroom.

Finally, apart from software bugs, single-purpose alarm clocks are quite cheap and reliable. Although it is not in my bedroom anymore, I still have an alarm clock that was my bedside alarm clock when I was in elementary school. It's thirty years old by now, yet it still works perfectly. The alarm function still works (although it's quieter than it once was). The display still is flawless (although it's dimmer than it once was). It's a simple, reliable device. My phone...isn't.

[0] https://www.theguardian.com/technology/blog/2010/nov/01/ipho...

There is a lot of value on your alarm clock not receiving messages and phone calls, and not having any other interface that isn't related to reading the time or waking you up at the correct instant.

Separation of concerns. Phones operate as a sort of monolith which is dangerous because if your phone fails, it takes down all of your services. Phones are also designed to be replaced fairly often which can mean redoing your alarms and so forth.

It's one of those things you really don't realize until your phone suddenly dies on you and you have to scramble to get a replacement.

I can't slam my hand down on the snooze button on my phone, not without risking a fragile and expensive device

My phone sometimes dies for some reason overnight, my alarm clock doesn't. No software updates, no[1] battery to die, etc.

[1] okay, there's a backup watch battery in there

I want an alarm that's independent from my phone. That way if I lose, break, or forget to charge my phone, I still have an alarm.

I only use my alarm clock in addition to my phone when I must not miss something really important.

Personally, I sleep a lot better when my phone is not next to my bed.