Ideally, as a means of abstracting and controlling changes between your build and deploy steps and the third party tooling. Bash scripts are either too fragile for sufficiently complex applications, and therefore unlikely to be reusable. Additionally, if the script is sufficiently complex, then you have an additional complex component that you have to maintain yourself. For many projects, this is a non-issue, but for others, it can be quite the ordeal.
Who said anything about bash scripts? There are other shell scripts in the sea. Also, shell scripting is much more universal, so you can use the same infrastructure to manage your web servers, your database servers, etc. A language-specific build system means that when you need to start adding components made in other languages, you need to reinvent a lot more tooling that should have been universal to begin with.
It would. I use fish shell here, yet I prefer writing my build tasks with plain JavaScript. Fly seems to be a thin abstraction to accomplish the same. Now, the most striking advantage is support for generators/co-routine style for async stuff and also promises right from the get go.
