Although there is magnetic field between the metal layers, there is no field outside the cable. Also, a coaxial cable is not affected by magnetic fields from other wires. One benefit of this relates to computers. A computer disk is magnetic. Magnetic fields can accidentally clear a computer disk.
Coaxial cables are much safer near disks.
Another benefit concerns very high voltage cables. Because one conductor surrounds the other, you cannot touch both at the same time. It is a way to reduce chances of electrocution. Argonne National Laboratory, Division of Educational Programs, Harold Myron, Ph.D., Division Director.