It is now good for nothing. The indication is that they have never found any good and sufficient reason to be one. Christ stated that Christians have two distinct roles salt and light. For one thing the function of salt is to preserve that which is good. Salt acts against corrupting germs.
Salt also brings out the taste.
So Christ said that Christians are to fulfill this role. We act against evil, against those forces that corrupt life. We live actively and dynamically against spiritual disease. Salt can also lose its usefulness. Instead of bringing out the full flavor of the Christian life, the professed Christian can dull the taste of life.