tomcat 5.5 logging

[pc leung]

http://tomcat.apache.org/tomcat-5.5-doc/logging.html
The above page shows that logging-log4j.jar and commong-logging.jar
needs to put in $CATALINA_HOME/common/lib.
 If I have two files of jar already in my struts webapp lib,
Why do I still need to place them in $CATALINA_HOME/common/lib?
 In addition, the tomcat doc web page shows the log4j.properties only,
is the common-logging.properties not necessary?
 Thanks