Well, the idea is that maybe the command 'go' is different on different build systems/targets and if you need to change it to something else (go-x86, or something like that) then it's going to be more difficult the way you wrote things.
But in a small makefile, it's a simple search-and-replace to move it out to a variable. Adding indirection slightly degrades readability, so better to wait until it's first needed.