I used to just use RunKeeper on my phone, because I also always have my phone with me for safety and music/podcasts, but there were a couple of reasons that I wanted a separate watch. The first was that since I do run/walk intervals, I always needed to have at least one earbud in to hear the alerts or I needed to run with the phone out and the screen on (killing battery life) so I could see the interval timers on the screen. Neither of those options is super great. The one earbud thing is awkward and sometimes I don't want them at all, and the phone out means I'm constantly looking down at my phone when I really want to just look around.
With the watch, it has a run/walk feature built in, and I can set it up to use vibrate notifications so that during runs/races where I don't want to use headphones, I'm not screwed.

The first and last place I really had this problem was at Tink last year (I got the watch as a "you did it!" reward to myself shortly after coming home). I realized at the start of the race that I wanted to run with my headphones off and just soak up the atmosphere, but to stick to my training plan I needed to have one of my earbuds in, and I was completely unprepared for that. It didn't even cross my mind that I might want to run without the headphones on. I spent some time early in the race trying to figure out how to just keep one earbud in and wrap the other one around in the strap of my sports bra to keep it on my head so I wouldn't lose them (I use bluetooth earbuds so they're not connected to anything). I also like just being able to start/pause/stop my run without digging my phone out of my pocket, turning it on, etc.
ATTQOTD: I track mileage, but I end up retiring them mainly on feel and tread wear. I run in Altra Olympus 1.0's at the moment and they don't make great casual shoes, although I have worn a retired pair for walking at Disney before. They have an extremely thick/firm midsole that does not flex much. I love the firmness in a running shoe, but I prefer my retired-because-I-didn't-like-them-for-running Asics Nimbus for general walking because they are soft and flexible.
I am actually trying to get a new pair of Nimbus only for walking/casual, which is a hard pill to swallow because they are $$. I would just get a previous model and save some cash, but I need the wide width, which only comes in one color, and I haven't found anyplace that still carries my size. I recently ordered a new pair from Zappos but wowza, the 18's run so small! I have an 11D in the 17's, and they are probably a half size too big, so I was going to order 10.5's next time, but then I read the 18's run small and order a half size up so I thought well ok I'll get 11 again, that should be fine since I should have gotten 10.5 last time. NOPE! I need an 11.5 for the 18's. My toes were right up to the end of the 11's.