Common Lisp the Language, 2nd Edition
This function is occasionally useful as an argument to other functions that require functions as arguments. (Got that?)
The object is returned as the value of identity.
The identity function is the default value for the :key argument to many sequence functions (see chapter 14).
Table 12-1 illustrates the behavior in the complex plane of the identity function regarded as a function of a complex numerical argument.
Many other constructs in Common Lisp have the behavior of identity when given a single argument. For example, one might well use values in place of identity. However, writing values of a single argument conventionally indicates that the argument form might deliver multiple values and that the intent is to pass on only the first of those values.