As the name implies it's for building stuff. Most (all?) packages that use C++ FFI with node-gyp need it. A popular package that needs it is re2.
Many newer packages bundle prebuilt native code as transitive dependencies, so build scripts are less needed than before.
Historically it was to accommodate packages like the original SASS compiler:
https://sass-lang.com/ruby-sass/
Other times it was to avoid shipping binaries due to, erm, safety concerns. The package would include code in a different language, which in turn would compile into a binary library or executable.