Capacitance grading why it used in cables?

A cable can be as long as 1m for electricity into your PC or up to a 100 kilometres to carry electricity further or telecommunications data over a distance. With a copper cable a sender (or generator) sends the power or signal to the receiver (or load) which expects to receive it in the same shape, if a little attenuated.

Capacitance is one of the (primary) line constants which will attenuate frequencies and cause signal shapes to be recovered at the far end in a different shape.

The best example is a square wave pulse being sent, with the higher frequencies attenuated more than the lower ones - typical of shunt capacitance between 2 wires. Attenuating the higher frequencies will round off the pulses and make them more difficult to capture reliably at the far end.