Well, they were the ones who collected, selected and protected the writings you read as the New Testament. They didn't write them, but without them we would have no clue which of the 200-plus scrolls represent the New Testament for sure.
The Apostles did not 'put together' the Christian canon; the Church did. The Apostles wrote it, and we believe that God inspired them to write the truth, as we believe that God guided the Fathers in selecting, recognizing, safeguarding and putting together that which the Apostles wrote into what you now consider God's Word of the New Testament.
As for the validity of the deuterocanonical books, one of the best reasons I can think of why the Church included them is the fact that the concept of the devil as the embodiment of all evil, and of a fallen angel who rebelled against God, comes from them and finds its place in the New Testament.
For sure, such is not the case with the Old Testament. The concept of a 'devil' does not exist in Judaism. The ha-satan of Judaism is a willing servant of God. The Jews deny that an angel albeit a noëtic being can have free will and rebel against God..
Thus, throughout Judaism, ha-satan is a loyal angel of God, who was created by God to tempt us. Jews often quote Job (probably one of the oldest books of the OT) where this ha-satan is portrayed as a member in good standing with God.
The concept that satan is the devil appears in the deuterocanonical books and from them in the New Testament, which means that the Apostles used them.
The early Church recognized the inspiration of the Old Testament as well as the New. The rejection of the Old Testament is, in part, a heretical view know as Marcionism.
My construction on reality is
that Holy Spirit preserved and decided the Canon.
He could have probably used . . .
donkeys, monkeys, rocks . . .
to do so . . . given His ability to raise up rocks as children of Abraham.
If it worked that way, isn't it curious that they are never quoted in the NT? I have no problem with the Apostles knowing the Septuagint, and I don't think a quote in the NT is necessary to validate an OT work. I just have a hard time believing that the NT presentation of satan is BASED on the deuterocanonicals. Inspiration means that the ultimate basis of the writings was God Himself, rather than other works.