This comment on HN does a great job summarizing Bashly.

…think of this as an argparse equivalent for Bash. You provide a YAML file listing commands, subcommands, arguments, and flags, and it automatically generates a Bash script that can parse and validate them, provide help messages, and run your code for each command.

It also lets you keep the actual code for each command and subcommand in separate files, which are merged together into one distributable Bash script at generation time. It’s basically a templating system to auto-generate argument parsing so you don’t have to solve that again or deal with things like optparse.