Well, necessary to what? To gospel mission.
I don't want to confuse anyone about my stance on this, but just as I'm concerned about any good works (whether it's feeding the poor or my having a quiet time) muddling the free grace in the finished work of the gospel, I'm also concerned about those of the brethren who somehow extrapolate that the cause of other-care is expendable.
These are just bullet points, but here's why I think social justice is a necessary component of mission.
1. God doesn't suggest we care for widows and orphans; he commands we do so. And not just a few times.
2. In the miracles of Christ we see signs of God's inbreaking kingdom, which is to say not just that they signify God's power in Christ's Lordship but that they signify that God's kingdom is restoring righteous order to the world. Acts of social justice, in much the same way, are these signs. The gospel changes the world.
3. In the letter to the Galatians, when Paul was confirming that "his" gospel was on the same page as the gospel of Peter, James, and John, those pillars reminded him to care for the poor, which Paul says is the thing he is eager to do (2:10). So even within the contextualized mission of the same gospel message to different cultures/tribes, care for the poor is a constant.
4. God is redeeming people, but he is also redeeming creation, which is outright groaning for its restoration. When the Christian enacts social justice for the glory of God, he is engaging in acts of eucatastrophe, redeeming the time in pointing to the Christ-shaped path back to the Garden. Christian social justice gives witness to the rightside-upness of God's kingdom.
5. When we are saved, we are changed from self-worshipers to God-worshipers, and as God equates love of our neighbor with demonstrated love of him, acts of social justice are proof of our redemption.
6. When you read through the Old Testament Law, you find an astounding amount of strictures not just on right relations with the community but also about meals, about work, about forgiveness of debt, about rest days, even about how to treat livestock. This tells us that there is a righteous order God expects his righteous people to live within. The work of social justice testifies to this order.
7. Caring for "the least of these" is caring for Christ. (Matt. 25:40)
8. For those who would say we should care for the poor, but for the Christian poor, not necessarily the poor of the world, I offer but two objections:
a. What do we do with the parable of the Good Samaritan, in which the "bad guy," the outsider, is made the hero of the story? This parable is given in response to the question "Who is my neighbor?" Doesn't this story, even if it were all we had as a clue of who to care for, say something radical about our scope of concern?
b. The mission of Jesus Christ was to love you and I while we were most decidedly not Christians. We were poor in spirit, enemies of Christ and his kingdom, and he offered his body for us anyway. Is it even Christian, then, to say we will only care for those who are like us? For some reason it doesn't occur to us to question foreign missionaries who give of themselves, even to the point of death, for the lost and the pagan overseas. But stateside we give the unsaved scraps from the table.
Yes, the riches of Christ is all the satisfaction a sinner needs, and we must never obscure or dilute that good news, even with works that are good. If silver and gold have we none, "such as we have" is nonetheless eternally precious. But if we have the silver and gold, shouldn't we give that too? Some may think that to be so free is to obscure the real pearl of great price. But in a delicious biblical irony, loving generosity doesn't show we think money is important, but rather that we find money cheap in comparison to the treasure of Christ.