When to use Flexible Metal Conduits vs a rigid conduit can be determined by analyzing your wiring situation and see which type of conduit will do the job best. It is primarily used in areas where a rigid conduit would be hard to set up as the Flexible Conduit will bend to match.
Although Flexible Conduit is a lot easier to use than rigid conduit, it offers less security for those conductors it conveys. Bending a stiff conduit may be an arduous task so it is often preferable to opt for the flexible selection. Being a metal conduit, it has an advantage over PVC conduit (which may not bend) since it might also function as the grounding route although some regional codes may require that you also run a green grounding cable.