Road biking challenges you differently aerobically, and will improve your fitness if approached correctly. Mincing around on any bike does nothing now does it?
Road Biking will not make you into a trail demon, it does little or nothing for the technical aspect of MTB’ing. The few events I’ve done however since I discovered a road biking club/interval training I’ve found I can turn the pedals longer and faster up fireroad climbs and have greater aerobic endurance than other MTBr’s.
If you ever go MTB with roadies as I did recently, you likely smash them on the trails, but they’ll smash YOU on a smooth climb.
The pro MTBr’s use road bike training for a reason.
Other than that, its all cycling, just slightly different. Don’t buy it if you can’t enjoy it.