The new engine should read these data files and, in theory, load and understand them in a way that is indistinguishable from the original.
Game engine recreations are made to allow the usage of classical games with newer operating system versions, recent hardware or even completely different operating systems than originally intended.
Another motivation is the ability to fix engine bugs which is often hard or impossible with the original engines (with notable exceptions, see community patch) once a software has become unsupported, with the source code not available.
In the development phase this has the disadvantage that for a long time no running prototype exists.
Static recompilation is another approach based on the original binary executable, potentially leading to better performance than emulation; an example is the 2014 ARM architecture version of StarCraft for the Pandora.