You shouldn't have to justify anything, but, it can be fun to try. I like to think of the AW like this...
For at least 12 years I have had a watch on my arm nearly every day except for a 1 year period of time I think of as the dark age where I tried just using my phone. OMFG, so many times I'd just be yanking my phone out just to remind myself of the time. What a dumb waste of effort and risk of dropping/tossing my phone every time.
Anyway, every watch I've had in that time has cost $350+ because that is how much you have to pay to get a decent looking watch with a link bracelet type band and also is something I like, style-wise. In the grand scheme of things, that's actually rather low end.
So compare to the Apple Watch: 1) I already feel no need to justify having a watch on. It's normal. 2) Its cost is easily within the normal realm of nice watch costs and every Apple Watch is nice, even that stupid rubber band, but, ignore that and we start at $649 for a very well made leather band and stainless steel case and it can tell the time. Guess what? You're already qualifying for many real world watches costing $649, but, it doesn't end there. You have every other feature and potential feature of the Apple Watch and the only real tradeoff is the battery life, and maybe the water resistance if that matters to you. That's an IF.
So, in reality, AW should not require any further justification than any decent low market watch. You could just stop there, and say something like "I felt like getting a new nice looking watch."
But if you feel, for some reason, you need more reasons but don't want to be a big salesjerk, you might consider adding something like "And I think this one may have some unique advantages. I felt like exploring what it might be able to do for me."
Don't try to convince others why THEY should want it, that's silly. Just tell them why you wanted it. "It looks nice and I think it might do cool things." what else do you really need to say? If you get prodded on the price, just bring up the cost of "dumb" watches and shrug. And if at any point they are trying to criticize this decision...? Maybe they need to screw off and mine their own damned business.