Eleven others, besides Joseph Smith have testified of the reality of the gold plates from which the Book of Mormon was translated.
I find their testimony important for a few reasons.
If the witnesses had not borne testimony, then Joseph Smith would have been the only person to have seen and witnessed the plates (assuming that those who helped him transcribe the Book of Mormon never saw it … including Emma Smith). If that were the case, then there is only one leg on which the Church stands. You either believe Joseph or you don't … it's that simple. And if Joseph were the only witness of the gold plates, then all the stories about how he wrote the Book of Mormon and how he made it all up would be believable. There would be many ways to explain how he wrote the Book of Mormon if there were no other witnesses.
But he is not the only witness. Eleven others put their lives and reputations on the line in order to testify of the reality of the gold plates. Now, explaining away the gold plates and the whole story of how the Book of Mormon came to be becomes much more difficult and complicated.
"Well, maybe Joseph paid these witnesses or made them keep a secret about lying about the gold plates." Some might make this statement, but the reality is that even though several of the witnesses left the church and even persecuted the church, none of them ever denied the testimony they gave regarding the gold plates. And if anyone argues that the accounts about how none of the witnesses ever denied their testimony of the gold plates are simply made up by Church historians, well … that is a stretch and anyone making that argument is bent on finding any way to prove that the Book of Mormon is false.
The bottom line is that Joseph, Emma and the eleven witnesses all have a personal account of the gold plates and Book of Mormon. And if their witness is not enough, then the doubter can always read the book for himself and apply Moroni's challenge.
Some things have to be taken on faith … as in "that's the way the Lord intended it to be." But the Book of Mormon is not one of those things. There are plenty of witnesses and ways to arrive at a conclusion as to whether the Book of Mormon is true or not.