I think you may be confusing or miss under standing the breakout boards (Bob) your seeing. Some bob's use the Usb to take 5V power for the various features they provide, (Opto-isolation etc) but they still use the LPT for sending/recieving signals to control software.

If your seeing cards that only have USB then they will most likely be motion control devices which eliminate the need for LPT.
Often these cards still need a BOB to make connections to inputs & outputs( I/O's ) easier. The advantage of a motion control device over the LPT is much cleaner and stable signals are sent to the drives resulting in much smoother operation and often higher Acceleration/speeds can be achieved. The signals produced are much more stable so far less errors from missed or dropped steps, esp at higher feed rates.
The over advantages to motion control devices is they take all the work away from the Control software so tend to need much less computing power so can be used on cheaper computers and Laptops which with some control software like Mach 3 Laptops are a big No No.!!

Really the basic funtion of a BOB is to make I/O connections that bit easier and when using opto-isolated versions that bit safer for the computer.
Decent Bob's provide over features like, relays for turning thing on/off or like the one you linked to they provide PWM or analog ouputs for directly controling the speed of spindles when attached to VFD's etc.
Other feature are providing charge pump circuit so the motors etc don't recive any signals untill the control software (Ie Mach 3) has full control therefore machine starts up in a safe state.!

While Not advised it is possible to not use a BOB at all and wire direct to the LPT cable.!!