What is the point of amphibians of having gills and lungs?

Not all amphibians do, some salamanders are live-bearers and some frogs lack a larval stage, they leave their egg as a small frog. Most amphibians do have a larval stage with gills, some will never drop the gills (neonetism) and most frogs and some salamanders will develop lungs. Most salamanders dont have lungs and breathe through their skin. There is no specified reason for having gills and lungs at different stages of their life cycle; apparently it is a good strategy. If it wasn't, frogs would be extinct ;)