Samsung has always modified the stock Android browser to its liking. On the Galaxy S II, the browser was very heavily modified, being faster and smoother than the AOSP version. It had tiled rendering, independent brightness settings, and a lot of other great features. Samsung’s browser has, in my opinion, been the best Android browser out there.
However, Google has replaced the stock browser with Chrome. Android phones will no longer include the stock browser (it’s a shame too, Chrome is awesome but still laggy). Thus, it’d be logical for Samsung to create their own browser to include on their phones. It’ll probably be based on the open source WebKit software, since it seems that Samsung is hiring WebKit professionals at the moment.
The browser will just be another alternative Samsung makes to Google products. Besides the browser, they already have Samsung Apps, Music Hub, Video Hub, and other services installed alongside Google ones. Samsung is trying to create their own ecosystem, and a browser is the next logical move.