Orcs have no magic. Warcries.

These are two different things; how can magic be replaced with rage...? I don't quite understand.

Indeed, it's strange. If we're drawing an analogy, then rage replaces overwhelming magic, gating, counter-attack, and so on. But warcries are meant to replace magic; they were created so that an orc doesn't just mindlessly attack in melee during its turn. Otherwise, everyone would just max out Devil's Strike because there would be no alternative.