Why we use add_filter in wordpress

Question

I've read tutorial from codex.wordpress.org but I could not understand why it is used in wordpress. Thanks.

Answer 1

add_filter is used to modify content/data prior to display.

You can filter any of the hooks at http://codex.wordpress.org/Plugin_API/Filter_Reference. For example, adding a filter to the_content would allow you to tweak the content - perhaps something like adding Amazon.com affiliate codes to links to Amazon products.