Why is the law perfect converting the soul?
Preaching the cross without the law leading the way provides a sinner no context around why the cross is necessary. For the preaching of the cross is to them that perish foolishness; but unto us which are saved it is the power of God. 1 Corinthians 1:18 The law of the Lord is perfect, converting the soul: the testimony of the Lord is sure, making wise the simple. Psalm 19:7 Wherefore the law was our schoolmaster to bring us unto Christ, that we might be justified by faith. Galatians 3:24 What shall we say then? Is the law sin? God forbid. Nay, I had not known sin, but by the law: for I had not known lust, except the law had said, Thou shalt not covet. Romans 7:7 Jesus used God's law to show how someone can be in violation of it even when they think they are not. Whether or not they accept the free gift of salvation at least they are made aware of what and who they are rejecting. And when he was gone forth into the way, there came one running, and kneeled to him, and asked him, Good Master, what shall I do that I may inherit eternal life?  And Jesus said unto him, Why callest thou me good? there is none good but one, that is , God.  Thou knowest the commandments, Do not commit adultery, Do not kill, Do not steal, Do not bear false witness, Defraud not, Honour thy father and mother.  And he answered and said unto him, Master, all these have I observed from my youth.  Then Jesus beholding him loved him, and said unto him, One thing thou lackest: go thy way, sell whatsoever thou hast, and give to the poor, and thou shalt have treasure in heaven: and come, take up the cross, and follow me. Mark 10:17-21 We really should be motivated to reach the lost. And of some have compassion, making a difference:  And others save with fear, pulling them out of the fire; hating even the garment spotted by the flesh. Jude 1:22-23 God is certainly motivated and wants no one to perish. The Lord is not slack concerning his promise, as some men count slackness; but is longsuffering to us-ward, not willing that any should perish, but that all should come to repentance. 2 Peter 3:9
Do not be mistaken, this is not saying the law saves, as only the fulfillment of the law through the finished work of Jesus on the cross can save.
The law shows why we need the cross, it's really that simple, but oh so important.
The law tells us what sin is.
It reveals the wretched condition of a soul when confronted by the perfectly holy God of all creation.
The law of the Lord is perfect, converting the soul...