Metaphors can relate unfamiliar issues to familiar ones in order to make them clearer to readers. For example, if you were writing about economics for an audience of homemakers, you could contrast supply and demand with food in the refrigerator: "The less salami there is in the fridge, the more the kids argue over it." By showing the similarities between an unfamiliar situation and a familiar one, a metaphor can make the unfamiliar one more familiar.
Writing doesn't necessarily have to be 100 percent precise all the time. Indeed, it is often better to give a feeling or emotion in order to describe something, as people relate to these better than they do to quantitative descriptions. So, rather than saying "his room was above 32 degrees but certainly lower than 50 degrees," you could write "his room was an icebox." He doesn't actually live in an icebox, but by describing it this way you can make it clear how cold he was.
Metaphors make your writing more engaging. People have a hard time focusing on strict, exact descriptions that follow a "subject, verb, object" structure. By adding metaphors, you are making your writing more fun to read and encouraging people to stick with it.
Metaphors make readers think. By changing the topic they are reading and thinking about, they have to try harder to engage with what you're writing. This in turn encourages them to think harder about the topic you're writing about, and they will come away from it better-informed because metaphors encouraged them to extend themselves.