Electrical utility wire systems are built to endure a variety of weather situations, and therefore must be flexible rather than rigid. The root matrix of a tree secures a solid trunk and its branches to the ground, but allows for the tree to sway in strong winds. An electrical utility pole functions similarly to a tree trunk, but because it lacks a root system it must find flexibility elsewhere. This leads us to why electrical wires are strung loosely, rather than taught.
A tightly strung length of wire does not have the ability to withstand forces performed onto it by mother nature. Wind exerts force on the wire, which when transferred, sways in wavelike motion. The energy transfers to the poles, and dissipates to the ground. This ability to dissipate force decreases the likelihood of damage to electrical utilities caused by storm gusts.
In addition to wind, precipitation also exerts force upon an electrical utility wire system. Precipitation exerts gravitational force on the wires as it accumulates. In the winter, wires may hold the additional weight of snow accumulation. In other seasons, rain, adds weight to the wires and pulls them downward. A tight wire will stretch horizontally, allowing accumulated precipitation to settle. A wire strung with slack has an upward opening parabolic shape. As precipitation accumulates, it slides or flows down the slope to the vertex where it is more likely to continue downward.
Since metals conduct heat and electricity, therefore, electrical wires are made up of copper, which is a metal.
Hence, if the wires would be tightly stretched, the contraction and expansion phenomena, which is caused by heat, that they undergo each day and night would cause them to crack.
When heat is applied to metals it provides energy to additional electrons the metals have. This energy makes electrons to move out of their original shell, hence expanding the atom structure.
When the temperature of the environment where the electrical wires are placed decreases, the metal the electrical wires are made up of is cooled and the electrons from metal atoms receive less energy therefore retracting to a more stable state and producing contraction of metal to a size that might be smaller than the initial size.
If wires were to be tightly stretched, the contraction and expansion caused by heat would cause them to crack.