A large existing C++ code base is one good reason. It’s possible to write wrappers for most of the more web-friendly languages but they are a pain to debug and

A large existing C++ code base is one good reason. It’s possible to write wrappers for most of the more web-friendly languages but they are a pain to debug and maintain for any complex API. So if you need a relatively lightweight web front-end for a complex C++ backend something like this may be a good choice. I still wouldn’t go the cgi route, or use makefiles for that matter.