The HomePod is basically a smart speaker for iPhone users who use Siri and Apple Music, since no such combination currently exists in the speaker market (and likely never will, since Apple doesn't open up Siri to third parties). It's really more to prevent them from defecting to spotify and other voice assistants. I don't have the HomePod (it's not available for sale in my country), but if I were to get one, the killer app for me is probably the ability to use Siri to play Apple Music, the same way I do so on my iOS devices.
The Apple TV is there for users who want a premium viewing experience. For me, the killer app is its ability to integrate with my other apple devices and services, allowing for some degree of consistency. The UI on my 5th gen ATV (the one with the A10x chip) is fluid and smooth, and I guess I am one of the few posters here who actually likes the remote for how easily it lets me scrub through content and scroll around. I am also going to assume that by using apps through the ATV instead of those bundled with smart TV, the TV manufacturers don't get my usage / viewing data, so there's that added element of privacy and security.
I do wish Apple would make a TV stick just for airplay mirroring. I have a couple of 3rd-gen Apple TVs in my school just to mirror my iPad to the projector screen, and I would love to be able to invest in a few of those sticks to bring around with me for presentation purposes.
As for the Apple Watch, I find it's really more of a bunch of conveniences rolled into one tight little package. I use my watch for notifications, Apple Pay, Siri, calendar events (via the Siri watch face), quickly viewing app data (like passwords stored in 1password), music controls and well, just telling the time. It really is like a remote control for your phone. For me, the killer app is being able to quickly see incoming notifications on my wrist, which means I don't have to fish out my phone every time it buzzes, as well as be informed of upcoming lessons (I have my timetable reflected in my phone as calendar events, which notifies me 15-30 minutes prior so I have enough time to get ready for my next class). When you are constantly on the move, every little convenience matters.