What is the need of rebuilding of an index?

          ANALYZE INDEX  INDEX_NAME VALIDATE STRUCTURE;

• Using above statement we can rebuild a Index.
• Each command creates a single row in the V$INDEX_STATS view. This row is overwritten by the next ANALYZE INDEX command, so copy the contents of the view into a local table after each ANALYZE. 
• The 'badness' of the index can then be identified DEL_LF_ROWS/LF_ROWS.