These kinds of projects are common when abandonware has large communities when the original developer has disbanded and can no longer support or update it, an effort is then made to get it running natively on newer versions of- and/or entirely different- operating systems and platforms. In that case, reimplementing that game alone would save more time than implementing the platform it runs on. This is especially true when a developer only wants to get one game (or a number of games that use the same engine) working.

Sometimes a full system emulator is overkill.