This bench folder contains different benchmarking files that you can run via astro-benchmark <bench-file-name>, e.g. astro-benchmark memory. Files that start with an underscore are not benchmarking files.
Benchmarking files will run against a project to measure its performance, and write the results down as JSON in the results folder. The results folder is gitignored and its result files can be safely deleted if you're not using them.
You can duplicate _template.js to start a new benchmark test. All shared utilities are kept in _util.js.