What's the point of a smart trainer?
When riding a virtual course on a smart trainer, if you begin to climb a hill the resistance ramps up and you have to pedal harder, just like when riding outdoors. This works with real-life POV video on apps such as Rouvy and FulGaz, as well as digital worlds like Zwift.
When doing group rides or races on Zwift, the resistance gets easier when you are riding in a group, simulating a draft, and when you are alone in the wind.
Similarly, when doing workouts on something like TrainerRoad or The Sufferfest, a smart trainer automatically sets resistance to the exact prescribed wattage, so you don't have to worry about gauging or pacing your effort.
So, there are key reasons to use a smart trainer: to make indoor riding much more fun and more productive.
How a smart trainer works
Smart trainers communicate on ANT+ and Bluetooth. ANT+ is cycling's standard wireless frequency, so most bike computers, heart-rate monitors, power meters, speed/cadence sensors and such work on ANT+. Bluetooth is native on smartphones and many tablets and computers.