For me, Siri when holding my phone is the least compelling use case. I kind of like using it for Alarm Clocks and Timers, because recognition in that domain is quite reliable, and one instruction saves multiple clicks.
Since touch navigation on a watch is less convenient than on a phone, some further use cases become more convenient with voice than with touch, e.g. asking "Is it going to rain today" before stepping out the door.
On Homepod, music is an obvious use case, which is unfortunately a rather difficult domain, because of the wide variety of media names. I use commands like "play some John Coltrane", "Shuffle play list Aggro", but also "who's playing piano on this track" (availability is a bit variable) or "what song is this". Home control is also convenient.
With Airpods, music is again the obvious use case, but I also like using them for walking directions (because you can walk without having to constantly glance on your phone). They also serve as a "poor man's CarPlay" in cars not equipped with a suitable media system (With the transparency mode on AirPod Pros, I feel that they are not an undue safety risk).
CarPlay is one of my favorite use cases, because I can navigate, listen to music, and listen and respond to messages without having to take my eyes off the road. When stuck in traffic, I also like asking what my ETA is.