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.

xor_generic.go

// +build !amd64,!ppc64,!ppc64le

package cipher

func xorBytes(dst, a, b []byte) int { 
	// generic implementation if more specific is not available
}

xor_ppc64x.go

// +build ppc64 ppc64le

package cipher

func xorBytes(dst, a, b []byte) int {
	// implementation specific to ppc64 processor
}