LYNDEN - When Joseph Smith translated the Book of Mormon at times he did not physically have the golden plates with him and translated through revelation. Although he translated both the Book of Mormon and the papyrus that contained much of the Pearl of Great Price, he did so with the original documents. I have three questions about this.

Question 1) Why did he need the documents in the first place.

JOEL - If the plates and papyri were not found, what Joseph Smith wrote might be considered nothing more than something that came from his own mind (which many believe anyway). The plates and papyri are physical evidence for the benefit of those who believe, proving that the scriptures did indeed exist, as witnessed by several individuals who saw and handled the originals.

Question 2) why did he sometimes translate with the plates if he could do so without them?

JOEL - Only Jospeh Smith knows the answer to this question. At first he did use the plates, sometimes holding the interpreters over the characters on the plate, or by copying the characters down on paper, and then using the interpreters to translate.
Later he was able to translate without the plates even being present. Perhaps there was a growing or learning process involved with receiving the revelation; sort of like training wheels on a bike. As Joseph's faith and experience in the revelatory translation process increased, there was less need to have the plates present to support his faith in the work. But this is all speculation. Very little is known about the translation process itself. Few details can be found from comments made by Joseph's scribes and close associates, and because many of their comments were either second-hand and/or recorded from memory many years after the fact, we can't be certain how accurate or reliable they are.
Only Joseph Smith knew the actual process, and he declined to describe it in public. At a Church conference in 1831, Hyrum Smith invited the Prophet to explain more fully how the Book of Mormon came forth. Joseph Smith responded that "it was not intended to tell the world all the particulars of the coming forth of the Book of Mormon; and…it was not expedient for him to relate these things" (HC 1:220).

Question 3) What stopped him from translating more books that have been lost? (other then it was not the LORDs will)

JOEL - Which books would that be? We have all the scriptures we need right now. Joseph did spend many hours retranslating the Bible but did not really get a chance to finalize that before he was killed. Perhaps if he had lived longer he might have been able to work on more things.