Argumentative discussions can contain various types of metaphor, but these do not all fulfil the same function in discourse: while some seem to express part of the argumentation, others seem to be irrelevant. So far, argumentation research has not explained when metaphors should be considered relevant for argumentation. In this paper, insights from deliberate metaphor theory are combined with the pragma-dialectical theory of argumentation. Based on this framework, it can be explained that various forms of metaphor exist that have various communicative functions. Deliberate metaphors are metaphors intended to change the perspective of the interlocutor and thus may play a relevant role in a discussion. Non-deliberate metaphors have no such function. Using the concept of analytic relevance, the analyst can distinguish between relevant and irrelevant metaphors: If metaphors are used as a means to change someone's perspective on the issue at hand, they are relevant moves in light of the aim to solve a difference of opinion, whether they positively contribute to that aim or not.