go build, go install, and go test accept the -i flag. From go help build:

The -i flag installs the packages that are dependencies of the target.

For example, if you run go build -i example.com/a, and example.com/x imports example.com/y, go build installs the file $GOPATH/pkg/$GOOS_$GOARCH/example.com/y.a.

This was useful for speeding up builds before Go 1.10, since previously installed packages didn't need to be recompiled. go build re-used packages installed in $GOPATH/pkg.

Go 1.10 introduced the build cache, so the $GOPATH/pkg directory is largely obsolete. Compiled packages are now stored in $GOCACHE. The -i flag merely copies files out of the build cache, so there's no longer any performance advantage.

The -i can cause errors when a target directory is not writable, as in #37962. In that issue, VSCode installed tools with -i, which caused errors when $GOROOT/pkg was not writable (common when Go is installed system-wide with an installer). Something caused runtime/cgo to be recompiled (perhaps a new clang version or flag), but it couldn't be written to $GOROOT/pkg/darwin_amd64/runtime/cgo.a.

Because of these problems, we should consider deprecating and eventually removing the -i flag.