Resistance converts a part of the power flow going through the cable into heat, which increases the temperature of the cable.
A high temperature can make the cable more saggy and its distance to the ground may become very dangerous.
A very high temperature may damage the cable.
Last but not least, this power converted into heat represent a loss of energy. Power plants have to produce more than the actual energy demand because of these losses.
That is why it is advisable to decrease as much as possible the resistance of the cables as long as it makes economic sense.
hope it helps
