زَيْدٌ أَسَدٌ
Zayd is a lion.
In an appropriate context, “lion” expresses courage figuratively; the context rules out an actual animal. The person is presented directly under the borrowed image.
زَيْدٌ كَالْأَسَدِ
Zayd is like a lion.
The second formulation contains an explicit comparative particle and openly states a resemblance. The first places greater emphasis on a claimed identity with the image. Both constructions may be meaningful, but they differ in the specific rhetorical effect of metaphor.