biggest benefit for training is that you get stronger by muscling up hills (rathter than shifting down), and faster by spinning on the downhills (rather than shifting up).

Really not a lot of true training benefit over simply not-shifting your road bike, and always peddling. Most people dont have that discipline.

There are other advantages outside of 'training', though... most notably the simplicity of the machine (no maintenance) and the connection to the road (no coasting).

they typically cost less than a road bike because theres far fewer parts. you can even make one from one of your current bikes for under 200 bucks. or just set the deraileur limit screws and see what you think.