This macro allows specifying the test for CASE, e.g.,

(fcase string= (subseq foo 0 (position #\Space foo))
  ("first" 1)
  (("second" "two") 2)
  (("true" "yes") t)
  (otherwise nil))

is the same as

(let ((var (subseq foo 0 (position #\Space foo))))
  (cond ((string= var "first") 1)
        ((or (string= var "second") (string= var "two")) 2)
        ((or (string= var "true") (string= var "yes")) t)
        (t nil)))

If you use a built-in HASH-TABLE test (see Section 18.4, “Function HASH-TABLE-TEST”) as the test (e.g., EQUAL instead of STRING= above, but not a test defined using EXT:DEFINE-HASH-TABLE-TEST), the compiler will be able to optimize the EXT:FCASE form better than the corresponding COND form.