"Suppose a brother or a sister is without clothes and daily food. If one of you says to them, “Go in peace; keep warm and well fed,” but does nothing about their physical needs, what good is it?"
James says in verse 17 that dead faith is useless. And what is dead faith? It is a lack of compassion and concern for your brother or sister. It allows you to see suffering and simply go on your way. It makes you feel great about yourself while another goes without the things they need to survive. It tricks you into thinking you are alive when you are not. Dead faith walks past God's children and leaves them to starve.
And if the faith is dead, it is not good for anything. When batteries die, they need to be recharged or thrown out. Dead faith does nothing. It doesn't work. It bears no crops. It yields no harvest. There is no righteousness reaped. There are no works to be had because the faith cannot produce them. It is dead faith. Dead.
But if faith brings forth the fruit God desires, then it is alive. If it remains in the vine, then it will bear fruit. Works make complete the faith and brings it to maturity. Faith must be made complete in order for it to save. Faith must have works, else it is a dead faith and a dead faith is no faith at all. Faith is dead without works, so we need a faith that works!