Well, as I found out during a half marathon, a watch can do so much more and is so much more reliable when it comes to GPS tracking and pacing.
I used to rely on Runkeeper app on my phone to track my pace. For 10km and less, it worked fine but was a battery-drainer even without the 3G on. I attributed that to the iPhone's poor battery life but that wasn't the only reason I opted to switch to a running watch.
A part of me hoped I could get a PB for this run but even from the beginning, it was doomed attempt. Runkeeper app failed to start from the moment the race started. I only realised it midway through the second kilometer when I wanted to check my pace. Also, having to rummage around for the phone in my Spibelt during a run, not a wise idea. Anyway, upon checking, I found that the app had failed to start and with a groan, started it up again.
I ran and ran until I was at the 16km before checking my pace and to my annoyance, the app had crashed, my phone was close to overheating and the battery was teetering at 10%. Oh. My. God. This was when I decided I needed a running watch if I was going to take running more seriously.
I got my first GPS running watch a few months later. A Polar M400 and my sister hooked me up with trial heart-rate monitor to go hand in hand with the watch. Since getting the watch, I've used it for all my bootcamp sessions and runs. I tried using the heart-rate monitor for runs but found that the strap cuts into my skin like a saw. So I opted to run without the heart-rate monitor which works out just fine as I just needed to track my pace at running.
As for my phone, I still bring it with me on runs, tucked away into my armband. But now I only use it to take pictures and emergency calls if needed. My M400 last quite long on these runs with the furthest being 30km.
It's good investment if you are looking to improve yourself fitness-wise. GPS can be a little unstable at times (Garmin still takes the cake for that) but other than that, it tracks my pace, heart-rate, cadence and everything about my fitness just fine. So if you are here to get the answer to whether you should get a running watch. The answer is yes.
Just don't loose it in a pool of water at an obstacle course race.