JSPs can be completely portable between different applications and different servers. A disadvantage is that they have no particular application data knowledge, but this is only a problem if they require that kind of data.
One possible use for generic JSPs is for portable page elements such as navigation bars or corporate headers and footers, which are meant to be included in other JSPs. You can create a library of reusable generic page elements to use throughout an application, or even among several applications.
For example, the minimal generic JSP is a static HTML page with no JSP-specific tags. A slightly less minimal JSP might contain some Java code that operates on generic data such as printing the date and time, or that makes a change to the page's structure based on a standard value set in the request object.