1 Nephi 3:19

19 And behold, it is wisdom in God that we should obtain these records, that we may preserve unto our children the language of our fathers;

Here Nephi reveals to his brothers the reasons for, and importance of retrieving the brass plates. The first reason he gives is language preservation. Language tends to degrade (or progress, depending on your point of view) over time even with written records available. We can easily see that here in the United States. After just over 380 years (from 1620 – the pilgrims’ landing) we no longer speak English. We speak American, whose roots are English, but have been altered over time by lots of influences, such as immigration and slang. Those who don’t read the Bible, for example, have a hard time understanding that language. The King James Version of the Bible uses an older language that is not generally spoken any more. So some of the more subtle meanings tend to get lost unless you make it a subject of intense study that takes you into the culture and understanding the more archaic expressions used. Another consequence is that in an effort to modernize the language in scripture some mistranslations can occur, changing the intended meaning. By going back to the original KJV (and other writings of the time) time and again, we slow down the changes in our language.

The Mulekites left Jerusalem around the same time the Lehites did, yet in that same amount of time their language had changed to the point that when the Nephites discovered them (about the same amount of time since the Pilgrims arrived in North America), they could not understand one another. Both groups left Jerusalem at about the same time, so the only real difference was that Lehi brought records, and Mulek did not. (see Omni 1:17)