Jacob 4:1 - 4:5
Jacob 4:1 Now behold, it came to pass that I, Jacob, having ministered much unto my people in word, (and I cannot write but a little of my words, because of the difficulty of engraving our words upon plates) and we know that the things which we write upon plates must remain;
Jacob labored diligently in the ministry among his people. He has not written very much before it is so hard to engrave on the plates. But what he has written, he knows will be preserved because they are recorded on these plates rather than on paper, for example which would not last through time
Jacob 4:2 But whatsoever things we write upon anything save it be upon plates must perish and vanish away; but we can write a few words upon plates, which will give our children, and also our beloved brethren, a small degree of knowledge concerning us, or concerning their fathers—
Again writing on a non perishable material like gold will give his children, grandchildren, etc and to future generations a knowledge of the Nephites and of their history.
Jacob 4:3 Now in this thing we do rejoice; and we labor diligently to engraven these words upon plates, hoping that our beloved brethren and our children will receive them with thankful hearts, and look upon them that they may learn with joy and not with sorrow, neither with contempt, concerning their first parents.
Because Jacob knows that the record will survive over time and what it's influence will be to his descendents, he and others feel a very deep commitment to keep up the record. Even though the record records some very unpleasant events, they hope that their descendants will look upon the accounts of their ancestors as a positive sign that the Lord does love all his children and constantly works with them even if they are not as good as they should be. It should be a sign that the Lord never leaves anyone along.
Jacob 4:4 For, for this intent have we written these things, that they may know that we knew of Christ, and we had a hope of his glory many hundred years before his coming; and not only we ourselves had a hope of his glory, but also all the holy prophets which were before us.
The main object of these writings it to reveal to their descendants that knew all about Jesus Christ. That they know of His mission and what His death would accomplish for their benefit. In addition, that the prophets even before him and his brother Nephi also knew about Christ.
Jacob 4:5 Behold, they believed in Christ and worshiped the Father in his name, and also we worship the Father in his name. And for this intent we keep the law of Moses, it pointing our souls to him; and for this cause it is sanctified unto us for righteousness, even as it was accounted unto Abraham in the wilderness to be obedient unto the commands of God in offering up his son Isaac, which is a similitude of God and his Only Begotten Son.
The knowledge of Jesus Christ was had from the start of the world and all the holy prophets had His mission revealed to them. However, the Nephites as well as the other parts of the house of Israel are keeping the law of Moses because the performing the various ordinances required by the law all point to Christ. Carrying out the ordinances reminds them of their commitment to God and their responsibility to always keep Him in mind as they go about their daily activities. It will have the same effect on them as it did on Abraham when he was commanded to offer up Isaac.