والأصل في نحو الكاف
The established direct means of expressing a simile is an instrument such as comparative kāf.
«زَيْدٌ كَالْأَسَدِ» explicitly includes the instrument, whereas «زَيْدٌ أَسَدٌ» omits it. The latter expression must be interpreted from its context and rhetorical construction. Depending on the adopted analysis, it may function as an intensified or elliptical comparison, with further discussion required to separate it from metaphorical usage.
Omitting the instrument is not the same as omitting the point of resemblance. One deletes the marker of the comparison; the other leaves the particular shared quality unstated. Both can happen in a single phrase, but they remain analytically different omissions.