Some confusion may be due to my lack of knowledge on the subject. I (perhaps incorrectly) used the "DCC" term to cover all digital systems and "DC" to pertain to analog systems. I'll try to rephrase:
My understanding is that with analog (DC?) three rail, the outer rails are always the same polarity and the inner rail is always opposite in polarity. This functionality facilitated the "toy" segment (whistle, uncoupling, etc.).
Since digital systems function with two rail layouts, does the center rail still offer any functional or feature advantages over two rail?
Hope this makes sense.
Traditional Lionel and other 3-rail trains ran on AC power. In the mid-20th century, Lionel worked out how to send a DC power burst through the rails to activate a horn or whistle in the engine. You could run an AC engine on DC power, but if it had a horn, it blew all the time there was power on the track! You could add a bridge rectifier to the engine to replace the Lionel mechanical reverse unit, so on DC reversing track polarity changed the engine's direction, but horns and whistles had to be disconnected.
As noted, the three-rail track allows you to build layouts with reverse loops without needing to isolate the loop and add some way to reverse polarity like you have to do with two-rail track. In two-rail, one rail is positive, the other negative. In a two-rail reverse loop, the positive rail turns around and connects to the negative rail, which would cause a short-circuit. In three-rail, if the center rail is positive, then the two running rails are negative. Even going around a reverse loop, the center rail is still positive and the outer rails negative, so no special wiring is needed. But this works the same whether you're running AC, DC, DCC, or other systems like Legacy etc.
In an analog system, how much power is on the track determines how fast the engines on the track go - all engines in the same block will respond to the power. In a command control system, each engine has a unique ID and receives an individual signal from the controller telling it what to do, so one engine in a block can move while another stays still. In DCC, there is a constant AC power fed to the tracks, and the decoder in the engine picks up the instructions from the controller and converts the AC to DC to feed the engine's DC motor, lights, sounds, etc.