You don't. You can specify this in a single line. Unless the question was, why do I have to mark targets as PHONY more generally. In which case the answer is,

You don't. You can specify this in a single line. Unless the question was, why do I have to mark targets as PHONY more generally. In which case the answer is, you don't. You only do this when you want the recipe to always run when invoked, as opposed to the standard mode where invocation is conditional on the file being out-of-date.