This is, by leaps and bounds, the thing I use Siri for most frequently.
"Hey Siri, add toothpaste to the shopping list" throws it onto a Reminders list that's shared with my wife, so whoever happens to be at the store next always has an up-to-date list. It's shockingly convenient.
Now, if only Siri were smart enough to let me add multiple items in one go...