I had a few reasons, when I tried tackling RE of a DOS-era game a number of years ago. I wanted to document the file formats, look through the data and find unused media, identify and fix some of the more egregious bugs in the game, build modding tools for it, understand the techniques used to build something that was memorable from my childhood, reduce the friction of running it on a modern system, make available optional improvements like higher resolution, texture replacement, and so on.

It ended up being more than I had the patience to finish. It might be doable for me now.