The essential function of coaxial cables is to transmit high frequency energy and signals with low loss, with reflection less matching and without Phase and Amplitude distortion.
The problem in high-energy transmission is the losses because of
i) cable dielectric
ii) cable construction,
iii) radiation, and
iv) interference with adjoining circuits.
The coaxial cable construction has been invented to overcome these problems, in which signals and energy are contained within an enclosed space. It consists of a core conductor, surrounded by pure PTFE dielectric covered with a metal shield, and finally encased in an overall jacket. The shield provides the return path as well as the confinement for RF energy, and is generally grounded.