Jesus used motifs from day to day life of Israel to bring home divine truth to them by way of parables and metaphor.
Salt has a number of uses including that of tastemaker.Accordingly, it was prepared from various sources by using different methods. One such use of salt was to preserve the fire in kitchen. In those days, log served as the main fuel of kitchen . Once the supper was cooked, the embers remaining in the stove would go waste unless preserved.One solution was to sprinkle water over it and rekindle it for making breakfast the next day.Another method was to cover the left over ember with salt crystals. The crystals would not get burned out,and would keep the ember live till next morning.Over a period of time, the crystals would lose saltiness and shape. They would then be substituted with fresh salt and discarded on the street.
Similarly, salt used as a preservative say, of dry fish and dry meat, would also become worthless over a period of time and discarded.
The audience of Jesus was well versed with those practices, and understood in physical sense, what Jesus meant by alluding to salt losing saltiness.The spiritual meaning would definitely sink in.
As for the metaphor of light, it came from another practice . Match sticks had not not been invented, and it was necessary to preserve fire over the night. A small lamp with low flame would serve the purpose.But it ran the risk of getting toppled by domestic animals or mouse. Therefore is was kept under cover with proper ventillation, the flame being hardly visible from outside. Unlike the salt over the hearth, the flame had no possibility of becoming worthless, but it carried the risk of being self centred.
Thus, Jesus was presenting two distinct spiritual truths with the metaphors of salt and light.