Why not just run it in an emulator or VM? Is there tangible benefit to reimplementing everything, or is it just an intellectual exercise?

"high-resolution rendering and substantially more advanced lighting" and several other items are documented as improvements in the link.

A quick glance through the README would demonstrate the litany of improvements you can expect from a proper decomp/port, compared to emulation.

Yes, a lot of these are much improved by reverse-engineering them and adding a few tweaks to modernize them. Many of them have added support for different display aspects, since most retro games are tied to 4:3. I downloaded the new RE of Sega Rally the other day and it was nice to have full proper widescreen.