In Simple terms the road bike is about time, distance and efficiency for me, generally you’ll cover more ground, more quickly on a Road bike than you would on an MTB…
Originally I got a Road bike on C2W from my previous job as I wanted to cycle to work and on the MTB it took an age. I still commute to work on the Road as its far quicker, although now on a drop bar fixie, and I use the geared Road bike at weekends or in the evenings to crank out some miles when the local trails are swampy and/or I’m too pushed for time to get in a decent loop on the MTB.
If you’ve only got an hour or so you can in a reasonably ride on a Road bike, on an MTB an hour doesn’t get you very far IME…
Both are good exercise, but the same effort carries you further on a Road bike…
I think I’ve also come to appreciate Road bikes and riding far more in the last couple of years, it’s not better or worse than MTBing, just different, I’ll always be an MTBer, But I’d like to think the extra riding I do because I also own a Road bike means my fitness for MTBing is a bit better and I get more out of it…