Statistics represent the uniqueness for the indexes that are being used for selecting the records. This can make the query execution pretty efficient. The tables that we are dealing with if truncated and repopulated, there is a good chance that the indexes and statistics are out of sync and this is when we have to update the statistics. There are also other situations like when the table has been modified and lot of rows have been added recently or like when a server has been updated with different version of software. These also give us the reason to use the UPDATE_STATISTICS, DBCC SHOW_STATISTICS etc to update it accordingly.