>would you have opted in?
Yes.
>For what feature or benefit?
By the time I pull into my driveway my garage door has opened and the lights in my hallway have turned on.
If I'm working from home, my thermostat stays at its "comfy" setting. If I leave the geofence between certain hours during the week it goes into "ultra cheap-ass energy miser" mode.
When I am at a meeting, and I have another meeting in a different location, my Maps application checks the traffic to the route and tells me if I have to leave early due to congestion. This is especially useful for dentist appointments made six months ago that have slipped my mind.
When I turn on my car and my phone connects, it knows if I am likely to be heading to work or if I am heading home and will suggest the best of three routes to take depending on traffic.
If I am somewhere and have an idea, I hold my smartwatch up to my face and ask it to "remind me when I get to X to do Y" and then when I get to X a notification pops up to tell me to do Y.
I get hyper-local weather notifications. Just 5 minutes ago on my smartwatch a notification popped up saying that rain was starting. It then started raining. There have been times when I have been outside not expecting rain, had my watch say "oh man rain's a-comin!" and then I looked around at the clear skies and said "nuhuh... I looked at the forecast today and there's no rain" but then I went inside to check on the computer again and it started raining.
While driving I can just ask: hey give me directions to the nearest gas station, and it will. No looking at screens required.
>With what risks?
None. There are literally zero risks. Evil Ruskie hackers could haxxor all of my data and know that I go to work every (non-quarantined) day and go to Safeway every Sunday afternoon, and it would not change or endanger my life in any way whatsoever. Anything they could do with the data, they can do without.