I have never once had to make a build.rs file, except to make python bindings for some of my own rust libraries. Are people out here using build.rs for everyday driving?
Bindings, and generally any form of code generation, is the most common form of build.rs I see in the wild.
One such example of "daily driving" is for gRPC, where the canonical way to generate the protobuf and gRPC bindings using Tonic+Prost is through build.rs. Another is C-to-Rust bindings, for all of those -sys crates.
Though I've also seen it used to circumvent limitations of Cargo, e.g. for invocations of dependent cargo build steps, etc. Overall, it's less common (or more robust?) than it used to be 5-ish years ago.
