Actually, in all honesty it is done because of one reason:

Star Wars is a creation of George Lucas and HE wants to use today's technology. The filmmaking techniques that are available today can often trace their roots directly back to those developed to create Star Wars. It is up to him, and the only reason he keeps holding back from going all out on the OT is because of the fans that gripe about every little thing he does. He's trying to make everyone happy by doing a little of both, but is ultimately disappointing everyone.

The Wizard of Oz is an odd case there, because I don't think anything could be changed to make the film be better. Replacing the cowardly lion with a Realistic, CGI lion would not make the film better. Removing a walking trash can robot from Star Wars COULD improve Star Wars for the better.