When we say that somebody has “a heart of gold”, no one thinks that this is actually the case. But the phrase gives a clear picture of what we try to say. In this case, the tenor is the emphatic/loving person and the vehicle is the heart made of gold. The metaphor suggests that a person is rich of heart, and when the heart is often linked to emotions and relations, it is likely that the person in question is generous with these feelings. In that case a description can be difficult, and it can be hard to communicate the feeling you get from the metaphor in ordinary terms.
This links to the other reason we use metaphors, to make the language come alive. Particularly artists are known to use this tool, to capture the audience. One of the most popular Shakespeare quotes is actually a metaphor:
All the world’s a stage,
And all the men and women merely players;
They have their exits and their entrances;
And one man in his time plays many parts.
(As you like it. William Shakespeare, 1599)
In this case, the tenor is life and the vehicle is a play. Shakespeare makes the point that our life is a play, played out on stage with audience and that we all have several roles to fill out. And while it is true, the quote makes the play more alive to the person watching the play, it is a different way of viewing life, and it makes you think about what is being said. A metaphor conveys images and a feeling of the descripted tenor.
Last we have the dead metaphor. It is a metaphor that does not convey feelings, but more an image. It is often a word that is no longer the description of something