I've seen it mentioned several times to fix issues with some packages, that it installs newer versions of packages somehow. But I've been unable to find a clear explanation of what that option really does, when to use it, why some packages have a --HEAD version to install and why its necessary.
I found some information at this github wiki.
Quoting from under the "Advanced Formula Tricks" section:
Hopefully this helps with the what. I can't help with the why.
In git using the
When you sync from other tags that are published as "known stable" releases then the developer is saying that the files as they existed at that point will build and has passed all if its tests.
Running from head can be risky.