If you haven't read the introductory documentation, it talks about the Model 2+1 "idea".
Struts is considered Model 2 (Model 1 was spaghetti code JSPs and maybe servlets). Model 2 is an implementation of MVC (Model View Controller). The idea behind MVC is to separate, as best possible the Model (your business data) from the View (the presentation: HTML or XML or ...) from the Controller (logic for moving from one page to another (but not business logic).
Turbine adds another "layer" of separation. Turbine uses Velocity for processing templates. Templates are pretty much "pure" presentation layer with specific velocity like tags. This allows your HTML developers to play with the templates without any impact on the
Java code. Meanwhile, your "engineers" can happily code Java without impacting the presentation. The split is more delineated in Turbine.
If your developement shop is not large, meaning you are basically the one who designs the graphics, writes the HTML, codes the Java, and engineers SQL queries against the Database, then it may not be a real big deal.
If, however, you do have a clear delineation of roles in your IT shop, then Turbine just may be more attractive than STRUTS.
Turbine also takes an approach of generating as much of the code as possible for you.
Cheers,
Mike