Personally, I purchased one for the following primary reasons [at 30 years old]:
1. I work in a manufacturing environment where I am constantly moving around the facility and I often cannot feel my phone vibrate or ring (its also very loud in here) having the watch "tap" me will prevent me from missing calls from my wife or other emergencies when someone needs to get ahold of me. [She has a few conditions which I need to know about when she tries to contact me]
2. I have started to get more into fitness mode. I like the idea of having the heart rate sensor to ensure I am going at it hard enough (but not going overboard) and I don't care about GPS at all, I run the same area just different amounts. So this should be great to give me feedback about my walk/run/jog & I don't mind taking my phone along for GPS if I do biking because I can store it on the bike instead of on me.
3. Homekit. I am extremely hopeful for homekit to eventually be a big success. I am waiting for Home Automation products utilizing homekit so I can leave my phone in the other room on the charger when I get home and I can just tell siri what I want to turn on and off or want information on. It should be pretty awesome to do that without having my phone out or to interrupt whatever I'm doing on my phone to talk to siri instead.
4... (somewhat) Mutitasking. I'm hoping that two screens are better than one... for example if I'm playing a game on my phone, perhaps I can answer calls on the watch instead of interrupting what im doing on the phone. Same thing for texts for if I want a quick answer from siri, I don't have to interrupt my phone session.
These are my primary watch purchase reasons. Of course everyone will have different reasons but for me these are very compelling.
I got the Apple Watch Sport 42mm Space Gray. I won't bother with a more expensive or "stylish" one until a later revision.