What are the functions of an intercepting filter?

The functions of an intercepting filter are as follows:
It intercepts the request from a client before it reaches the servlet and modifies the request if required.
It intercepts the response from the servlet back to the client and modifies the request if required.

There can be many filters forming a chain, in which case the output of one filter becomes an input to the next filter. Hence, various modifications can be performed on a single request and response.