A Cask is like a Formula in Homebrew except it describes how to download
and install a binary application. To learn how to write a Cask, see
CONTRIBUTING.md. For a complete reference, see
CASK_LANGUAGE_REFERENCE.md.
The idea is for each Cask to encapsulate and automate the story of how a given application should be installed. Join us in building up a community-maintained collection of Casks that is striving to become the standard way that hackers install Mac apps.
Yes, yes, yes! Please fork/pull request to update Casks, add features and clean up documentation! Anything you can do to help out is very welcome.
It's also pretty darn easy to create Casks, so please build more of them for the software you use. And if homebrew-cask doesn't support the packaging format of your software, please open an issue and we can get it working together.
The whole idea is to build a community-maintained list of easily installable packages, so the community part is important! Every little bit counts.
The short answer to this would be: for the same reason that Homebrew does not install applications directly into /usr/local.
We don't know up-front precisely what files are going to be in the
dmg/zip/tgz/etc, so it's really helpful to have a place to dump all of them
safely, then iterate through and act on the files we care about. For an .app file
this may be symlinking it into ~/Applications or /Applications, for a .pkg file
this might be running the installer. For a Screen Saver it may be symlinking it
into the appropriate directory for it to show up in System Preferences.
The reason we implement this project on top of Homebrew was based on a belief
that their methodology for managing applications has a lot of merit. We'd
prefer to try and work things so that we can keep ourselves Homebrewy both in
implementation and idioms. Trying to manage all of ~/Applications would move
the project more towards a standalone system, which would mean reimplementing a
lot of the Homebrew stuff we lean on now.