It's about always available without having to grab or pull up anything, especially handy when your busy or your hands are full. Once you become accustomed to it, having to pull out your phone is just cumbersome to do basic things. You use the example of the weather how all you could do is reach down pull up your phone, unlock it and go to the weather app or Google Now to see the weather. That's annoying when I'm trying to get my kids ready for school and I'm occupied doing something else while wanting to know what the weather is like to know what type of clothes to dress my kids into. Having to stop to grab my phone to do that is making me stop what I'm doing and slows me down in the morning.
I see. All of this sounds like many minor conveniences that all add up in the end. Maybe I'm just not the kind of person who uses assistant software. I tried Kinect (kinda sorts the same I guess), Google Now is on my phone but I never use it and when Win10 launched I thought I'd use Cortana but I don't. Maybe the tech has advanced sinced then.
I have to admit that having an always listening microphone in the house sounds scary to me, but I think I'll try one of those things out once they're dirt cheap.