I have a hard time believing people use Siri on the iPhone and can't fathom using it on Mac.
I use Siri in the following ways on my iPhone (using the 'Hey Siri' phrase when it's plugged in as it's a 6, not a 6s where it doesn't required plugged in).
Stay at home Dad here who runs the household, my own little business and works for my wife's business as well:
1. When I'm baking or cooking which is always bulk (3 or 4 times the regular recipe quantities), I'll ask Siri to convert measurements, ask if certain ingredients can be replaced by something else and to set timers. Usually because I have my hands all mucked up in those ingredients so touching a phone would be a bad move lol
2. While driving. My vehicle has the blue tooth built in and I constantly send texts, field phone calls from clients (and friends/family) hands free while driving. The BT feature also will read texts back to me which is handy (and funny

It's not like it's 1 message after another constantly, but a few times a day for sure.
3. Calendar appointments - probably the biggest use I have for Siri. I make calendar notes ALOT. It's helped me become much more organized than I ever was. I still have to figure out if I can use Siri to set 2Do tasks in the app '2DO' because I have extensive lists. Again, wasn't very organized before so I've gone crazy with lists and it's helping me lol
4. I will ask it the weather or sports scores for sure occassionally. I'd say #2 and #3 would be the top uses for me.
So yup, I'm excited for Siri on a Mac although I don't expect it to be super slick right away. There will be growing pains.
Cheers,
Keebler