> What does $host mean?
In Go parlance, it would be the remote host where the repository is hosted, e.g. github.com, dev.azure.com, golang.org, etc.
> Why would I need that for a purely local project that is only created for my own use?
If nobody else is using your purely local project, and you're sure nobody will ever use it until the end of time, then I guess you could just use "~/src/$HOSTNAME/$USERNAME/$PROJECTNAME". Otherwise, it would be wise to setup a remote repository ahead of time.
Go has a strong opinion that, in this day and age of distributed computing, projects should be online-first, so they can be easily used as dependencies. One of the nice consequences of this opinion is that Go dependencies can just be specified in the import statement - e.g. using grpc dependency is just:
import "google.golang.org/grpc"
No need for pom.xml, requirements.txt, cmake.txt, or any other kind of dependency configuration. It just works (unless it doesn't, like with private repositories, in which case it requires some exotic configurations in ~/.gitconfig or ~/.netrc, but that's a whole other can of worms - for most public repositories I've used it works flawlessly).
> And what about grouping projects? E.g. "personal", "work", etc.
Assuming you only use one repository hosting service and have one username, all your personal projects would be under "~/src/$PERSONAL_HOSTING_SERVICE/$USERNAME/", and your work would be under "~/src/$WORK_HOSTING_SERVICE/$WORK_ENTITY/" or something like this.
> And where in the structure are languages?
It isn't. That's either a bug or a feature. If it's a bug, you could just do the whole thing by language, e.g. "~/src/go/", "~/src/java/", etc.