Firsthand accounts say that Joseph Smith used a seer stone to receive the translation for the Book of Mormon (emphasis mine):
Joseph Smith put the seer stone into a hat, and put his face in the hat, drawing it closely around his face to exclude the light; and in the darkness the spiritual light would shine. A piece of something resembling parchment would appear, and on that appeared the writing. One character at a time would appear, and under it was the interpretation in English. Brother Joseph would read off the English to Oliver Cowdery, who was his principal scribe, and when it was written down and repeated to Brother Joseph to see if it was correct, then it would disappear, and another character with the interpretation would appear. Thus the Book of Mormon was translated by the gift and power of God, and not by any power of man.
— David Whitmer, "An Address to All Believers in Christ" (1887), 12; Quinn (1998, p. 172).
The LDS website on the translation of the Book of Mormon corroborates this account of Joseph Smith looking into his hat at the stone, and states that the gold plates containing the original "reformed Egyptian" version of the text were sometimes nearby, but covered.
Joseph’s wife Emma explained that she “frequently wrote day after day” at a small table in their house in Harmony, Pennsylvania. She described Joseph “sitting with his face buried in his hat, with the stone in it, and dictating hour after hour with nothing between us.” According to Emma, the plates “often lay on the table without any attempt at concealment, wrapped in a small linen table cloth.”
These accounts make it clear that Joseph Smith was not looking at the golden plates when he was translating the Book of Mormon, and they were covered by cloth so that they were not visible to anyone. My understanding is that sometimes the golden plates were not even in the room when he translated them. It seems strange that after all the difficulty he went through to get them, the golden plates were ultimately unused during the actual translation process.
Given that multiple sources say that Joseph Smith derived at least a major portion of his translation of the Book of Morman by looking at the stone in his hat "hour after hour", why were the golden plates necessary in the first place?