What is the electrical benefit of keeping a running disparity on a bus?
First - synchronization, it is easy to spot the difference between zeros and ones if they both occur often.

Imagine you are in a tunnel, someone far away has a flashlight. You agree how long should a "one" and a "zero" last. If he keeps sending 0101010101 it will be very easy to count all zeros and ones. It should also not be that hard if the pattern is 001001001. But if 1000 ones are sent in a row, how can you be sure that it is 1000 and not 999 or 1003? Even if you both had atomic clocks, they would drift apart. Sending an approximately equal amount of zeros and ones helps solve this problem.

Second - coupling. It is often beneficial to galvanically isolate the devices from the communication circuit (long cables, lightning etc.). Such isolation is often done with a transformer (eg. Ethernet) and transformers can't transfer DC. A transformer may pass a couple of ones in a row, but then the core will go to saturation and the signal will be lost.