At it's simplest, a breakout board just splits the parallel port pins into screw terminals.
But as you've seen, many breakout boards add additional features and functionality.
Most have opto isolators, to prevent damage to your PC from shorts, or wiring mistakes.
Some have circuitry to convert a PWM signal into a 0-10V analog signals for speed control.
Some have relays on them.
Some allow 3.3V parallel ports to work with 5V signals.
Some allow you to use 24V signals with your parallel port.
Some are designed to use multiple ports.
Some are designed to work with specific motion controllers.
Breakout boards can do a lot of different things. But they all basically make it easier to connect your components to the parallel port, or motion controller.