There are many reasons for using the jk2 protocol.
It means that you can expose Apache to the web, and have it serve up your static content (ie javascript, html, images), while passing calls for JSP or servlets to Tomcat.
Apache is quicker than Tomcat at serving static content because it is written in C.
You would not want to expose Apache and Tomcat to the web and do redirects from Apache to Tomcat because this exposes two servers - which increases security problems. Also, arguably because jk2 is a slimmer protocol than http, it would be quicker to use jk2 rather than using redirects.
There are other pros/cons of using jk2 & Apache as a front end to Tomcat, just google or go to the jk2 home page (again, google it).
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