"These buildings, different from the thousands of regular Church houses of worship scattered over the earth, are unique in purpose and function from all other religious edifices. It is not the size of these buildings or their architectural beauty that makes them so. It is the work that goes on within their walls. 
 
The designation of certain buildings for special ordinances, as distinguished from regular places of worship, is not new. This was the practice in ancient Israel, where the people worshiped regularly in the synagogues. Their more sacred place was, first, the tabernacle in the wilderness with its Holy of Holies, and then a succession of temples, where special ordinances were performed and where only those who met the required qualifications could participate in these ordinances. 
 
So it is today. Prior to the dedication of a temple, The Church of Jesus Christ of Latter-day Saints invites the public to go through the building and inspect its various facilities. But when it is dedicated it becomes the house of the Lord, vested with a character so sacred that only members of the Church in good standing are permitted to enter. It is not a matter of secrecy. It is a matter of sanctity." 
 
http://www.lds.org/temples/purpose/why/0,11581,1953-1,00.html