If we say that a film company has an Internationally-acclaimed back catalogue of features, what's the meaning of this "back" here?
Kind of "business" language meaning the catalogue is rich and backs the prestige of the company?
Yes and no. It is rich, but it's got nothing to do with prestige - except in that it adds to the company's prestige because of its richness.
I think publishers used the term 'back-catalogue' before film companies did. A publisher makes money both by selling newly-published books and by continuing to sell books that it has published in previous years - that is the back-catalogue. I imagine in the film world a company's back-catalogue is made up of films previously made. If you go to see Doctor Zhivago, that is part of MGM's () back-catalogue.