The phrase "must also be" has the sense of "it being necessary." Paul understands factions as God-ordained because he could see the pattern of them from Old Testament times, as well as the benefits derived from them.
The Corinthian congregation was a troubled group divided into factions by heresies (I Corinthians 1:10-13). This circumstance was not helping the already-calamitous situation, but Paul says that the calamity would eventually produce a good result. The true sons of God would be revealed by their reactions to the false teaching. They would not accept it, and thus would be witnesses to the weaker for the truth of God.
John W. Ritenbaugh
God's Sovereignty and the Church's Condition (Part Two)
Contrasting the unity that Christ died to establish is the division sown by the Adversary as part of his “divide and conquer” strategy. Paul writes of factions (or divisions) in relation to the members of the Corinthian church of God:
For, in the first place, when you come together as a church, I hear that there are divisions [schisma] among you. And I believe it in part, for there must be factions [hairesis] among you in order that those who are genuine among you may be recognized. (I Corinthians 11:18-19)
In general, the nouns schisma and (to a lesser extent) hairesis receive pejorative treatment in the New Testament. The thrust of the apostle's comments on the topic of church factions is that they showcase the presence of human nature at work in Christians. Schisms are the result of carnality alive-and-kicking in the church. They emerge where God's people are not fully committed to living His way of life, where they deny God His rightful place as Sovereign Ruler of their lives. Axiomatically, the lack of unity in a congregation reflects a proportional lack of repentance on the part of its members.
Paul introduces the concept of division, using the same Greek word as he does in I Corinthians 11:18, schisma, in the book's first chapter:
I appeal to you, brothers, by the name of our Lord Jesus Christ, that all of you agree, and that there be no divisions [schisma] among you, but that you be united in the same mind and the same judgment. For it has been reported to me by Chloe's people that there is quarreling among you, my brothers. (I Corinthians 1:10-11)
Quarreling is a manifestation of the unity-dissolving “friction or competition.”
Unity and Division: The Blessing and the Curse (Part One)
Other Forerunner Commentary entries containing 1 Corinthians 11:18:
1 Corinthians 11:18-19
1 Corinthians 11:23-29