Example: Sometime we need to show content on frontend with adding some prefix or suffix or may be you have enter spelling mistakes in 2000 posts. so it is very difficult to modify 2000 posts so we can modify using filter . SO mistake will be replaced by correction.   For Modifying post content only:  

function link_words( $text ) {
   $replace = 'Replace with';
   $text = str_replace( 'Want To replace text', $replace, $text );
   return $text;
}
add_filter( 'the_content', 'link_words' );

  For Modifying post excerpt only  

function link_words( $text ) {
   $replace = 'Replace with';
   $text = str_replace( 'Want To replace text', $replace, $text );
   return $text;
}
add_filter( 'the_excerpt', 'link_words' );

  For Modifying post content and excerpt both  

function link_words( $text ) {
   $replace = 'Replace with';
   $text = str_replace( 'Want To replace text', $replace, $text );
   return $text;
}
add_filter( 'the_content', 'link_words' );
add_filter( 'the_excerpt', 'link_words' );