People do it to give themselves more opportunities to succeed. As much as most of us in here love series, but actors in them that don't make it to movies can't go internationally and make it real big most of the time. There's always the acception but that's rare, it's the movies that get translated into several languages through dub or sub which broaden their chances of being recognized. Not only that international reason but people always believe that if they can do well in both or at least really good in one area then they can gain popular within their own country and that always lead to more money.