My reasons to use an outdoor GPS device (I have two different Garmin devices):
1) durability - My older Garmin is almost 10 years old - it has survived many treks, hundreds of caches, navigation on high seas, it was submersed in streams, rivers, seas and swamps, it fell down cliffs, rocks and trees, it was even run over by a car. No smartphone can survive that for so long, no matter what case you use.
2) accuracy and reliability of navigation - No phone is as accurate and reliable as a good dedicated GPS navigation device, certainly not the fancy and expensive Samsung I used to have, certainly not my girlfriend's iPhone.
3) power source - You can get spare batteries almost anywhere, anytime. Feeding a phone might be a problem on longer trips. You do not exhaust your phone by using the navigation and you do not exhaust your navigation by using your phone.
4) reliability of software - A smartphone is a very complex little computer with many uses - it also means it is prone to the same problems the big computers have. It can fail in various ways, get infected by a virus, go through various software glitches. It happened to me many times - and if it happens somewhere in the woods, it is a problem that is not easy to solve.
That said, I use a smartphone a lot and carry it all the time. For quick and easy caches, for most geocaching in my home city, a smartphone is good enough and more comfortable to use. But for any longer trip, especially for longer outdoor trips, in difficult terrain, sleeping outdoors, without access to electricity, a good outdoor GPS device is absolutely essential.