Justification by Faith alone is to distinguishing the Law and the Gospel

 By saying that the law and gospel distinction is a secondary matter, one is leaving the door open for the idea that one is justified by faith and works or faith and the law of obedience. The gospel is not a secondary matter bit as Paul says it is a matter of first importance - "For I delivered unto you first of all that which I also received, how that Christ died for our sins according to the scriptures;" 1 Corinthians 15

By knowing the law and gospel distinction believers are better equipped to understand and ground their assurance rightly by what Christ alone did.

Again, the elect are justified by faith alone. What is this faith alone if it doesn't distinguish between our sins which are many, which procures the wrath of God outside of Christ, and the only righteous thing that makes the elect acceptable before God?

If the law and gospel is merely a secondary matter then Paul would not have said did we receive the Spirit by the works of the law or did we receive the Spirit by the hearing of the word through faith?

If one gets the law and gospel distinction wrong they get God's act of justification wrong and vise versa. They two are intertwine. For by faith the elect believe the gospel and so are justified.