Precompiling the JSPs when you "build" the application is very important for user interface response times. If the JSPs are not precompiled, the application servers compile the JSP on-the-fly the first time it is used. These compiles can take up to 30 seconds or more and could lead to the perceptions of a badly performing system.
Please see WebLogic tuning: JSP pre-compilation on precompiling JSPs in WebLogic application servers.
Please see WebSphere tuning: JSP pre-compilation on precompiling JSPs in WebSphere® application servers.