Book of Mormon: 2 Nephi 12
2 And it shall come to pass in the last days, when the mountain of the Lord’s house shall be established in the top of the mountains, and shall be exalted above the hills, and all nations shall flow unto it.
3 And many people shall go and say, Come ye, and let us go up to the mountain of the Lord, to the house of the God of Jacob; and he will teach us of his ways, and we will walk in his paths; for out of Zion shall go forth the law, and the word of the Lord from Jerusalem.
So first of all, verse 2 is describing the Temple. Have you ever noticed how many of the Prophets in the scriptures would go up on a Mountain to talk to God? Well that symbolizes the Temple being the closest place to Heaven on this earth, or in other words, the Temple is the closest place to Heaven on this earth.
I love it how it says in verse 2, how all nations shall flow unto it. When President Gordon B. Hinckley (I named my son after him :) was acting as a Prophet on this earth, he established many Temples throughout the world and today many Temples are continuing to be built and dedicated. What a wonderful thing that "all nations" will be able to go to the Temple.
But why do we need to go to the Temple??
Well in verse 3 it talks a little about why it is important to go to the Temple. It says, "he (the Lord) will teach us of his ways, and we will walk in his paths". So we can learn.
My favorite part of verse 3 though is when it says "for out of Zion shall go forth the law". What does Zion mean?......Well peace. Do you want peace in your life? Well then you need to go to the Temple, learn the laws that equal peace, and then walk in His path! How cool is that?!
***Just a side note. It reminds me of science. When there is a law in science it means that whenever you do a certain action, it will always create the same reaction. This is no Theory. The Temple teaches the law of peace. Sorry, I never realized I was a science nerd but I guess I kind of am.
So to really answer the question, "why do we need to go to the Temple?" .....Peace.