Another use case for build tags is writing an optimized implementation of a function that takes advantage of a specific architecture.
For example a Go standard library uses this technique for implementing optimized xor functionality.
// +build !amd64,!ppc64,!ppc64le
package cipher
func xorBytes(dst, a, b []byte) int {
// generic implementation if more specific is not available
}
// +build ppc64 ppc64le
package cipher
func xorBytes(dst, a, b []byte) int {
// implementation specific to ppc64 processor
}