For me Alexa is great for Home Automation (which while turning off the lights is a night party trick, it's pretty great to turn down the lights when you're already on the couch. Yes, 100% unneeded, but it is nice.), Cheap Sonos Style Music in Every Room sync'd, and yes, ordering stuff from Amazon. Again, totally unneeded, but for certain goods, I've found it nice to just say, "Alex, order me more blah" and it replies, "These that you recently ordered?" "Yep" "Ok. Done."
All of this isn't stuff you need, but hey, it's pretty rad nerd stuff.
Its most useful features so far have worked best in the bedroom.
Alexa, set my alarm for 7AM. Wake me up in 2 hours. what's the weather in X? What's my commute? Nightlight.
That's about it.
edit: I started this before you edited in your examples, so sorry for the overlap :-)
1. Play music
2. Timers, especially when cooking
3. Turn lights and heating on and off. It's nice to be able to control them while in bed with just your voice.
Nothing I couldn't live without, but it adds convenience.
Not just that, but you don't have to hunt for apps and fiddle with UI. Asking Alexa to set the heat to 21.5 degrees is way faster than finding the closest mobile device, unlocking, launching the app and doing a bunch of taps.
While I'm missing a couple of skills in Canada (stupid region locks!), the smart home control has been fantastic for our household. We also find the shopping list helpful.
So yes, a $40 timer. Still worth it.