I frequently use Siri to create reminders during my commute, because it's more convenient than pulling my phone out (especially in winter!) and I almost always have an earbud in anyway. If a quicker, lower-friction method exists of capturing stray thoughts before they get away, I haven't found it yet.

I also occasionally use it when I wake up in the middle of the night and don't want to abuse myself with a bright phone display in order to find out what time it is - I don't keep a clock on my bed table, but my phone always charges there. "Hey Siri, what time is it?" is handy in that circumstance, unless the "Hey Siri" part decides not to trigger, which happens often enough that I don't really find it reliable. (I think that must be because a half-asleep voice sounds different; this isn't a problem I generally have at other times, even when using a $15 Bluetooth earbud which can't possibly have a very high-quality microphone.)