Question: Why were the gold plates needed at all if they weren't used directly during the translation process?

Book of Mormon Central, KnoWhy #366: Why Were the Plates Present During the Translation of the Book of Mormon? (Video)

Joseph did not need the plates physically present to translate, since the translation was done by revelation

Much is made of the fact that Joseph used a seer stone, which he placed in a hat, to dictate the text of the Book of Mormon without viewing the plates directly. ^[1]

Joseph Smith translates using the seer stone placed within his hat while the plates are wrapped in a cloth on the table while his wife Emma acts as scribe. Image Copyright (c) 2014 Anthony Sweat. This image appears in the Church publication From Darkness Unto Light: Joseph Smith's Translation and Publication of the Book of Mormon, by Michael Hubbard Mackay and Gerrit J. Dirkmaat, Religious Studies Center, BYU, Deseret Book Company (May 11, 2015)

Some witness accounts suggest that Joseph was able to translate while the plates were covered, or when they were not even in the same room with him. ^[2] Therefore, if the plates themselves were not being used during the translation process, why was it necessary to have plates at all?

Joseph did not need the plates physically present to translate, since the translation was done by revelation. The existence of the plates was vital, however, to demonstrate that the story he was translating was literally true.

The existence of the physical plates attested to the reality of the Nephite record

If there had been no plates, and Joseph had simply received the entire Book of Mormon through revelation, there would have been no Anthon visit, nor would there have been any witnesses. The very fact that plates existed served a greater purpose, even if they were not directly viewed during all of the translation process.

The plates served a variety of purposes.

1. They were viewed by witnesses as solid evidence that Joseph did indeed have an ancient record.
2. Joseph's efforts to obtain them over a four year period taught him and matured him in preparation for performing the translation,
3. Joseph's efforts to protect and preserve them helped build his character. If Joseph were perpetrating a fraud, it would have been much simpler to claim direct revelation from God and forgo the physical plates.
4. Joseph copied characters off the plates to give to Martin Harris, which he subsequently showed to Charles Anthon. This was enough to convince Martin to assist with the production of the Book of Mormon.

The plates' existence as material artifacts eliminated the possibility that Joseph was simply honestly mistaken. Either Joseph was knowingly perpetuating a fraud, or he was a genuine prophet.

The existence of actual plates eliminates the idea that the Book of Mormon was "spiritually true," but fictional

Furthermore, the existence of actual plates eliminates the idea that the Book of Mormon was "spiritually true," but fictional. There is a great difference between an allegorical or moral fiction about Nephites, and real, literal Nephites who saw a literal Christ who was literally resurrected.