Those are two very powerful metaphors that remind us of the potent roles we play in shaping way other people experience life. Jesus' first and primary concern was not to instill the right doctrine in his followers or to weed out heresy. The first thing he did was to help his disciples realize the importance of the impact they had on others. As Christians, we are the seasoning that God uses to bring out the best flavors of life, that others might taste its goodness. We are the light that God uses to illuminate what is beautiful and precious in the world.
Life is full of tragedy and loss and suffering and sorrow.
Everyone knows what it is like to experience those things. But without us, they may never know how good life is. They may never taste its love, its joy, its tender mercy. They may never see it's beauty, its kindness, its abundant generosity. God uses us to season and illuminate all the goodness of life.