Is it farther or further?
late 14c., "taking words in their natural meaning" (originally in reference to Scripture and opposed to mystical or allegorical), from Old French literal and directly from Late Latin literalis/litteralis "of or belonging to letters or writing," from Latin litera/littera "letter, alphabetic sign; literature, books" (see letter (n.1)). Meaning "of or pertaining to alphabetic letters" is from late 15c. Sense of "verbally exact" is attested from 1590s, as is application to the primary sense of a word or passage. Literal-minded is attested from 1791.
A constant made available to a process, by inclusion in the executable text. Most modern systems do not allow texts to modify themselves during execution, so literals are indeed constant; their value is written at compile-time and is read-only at run time.
In contrast, values placed in variables or files and accessed by the process via a symbolic name, can be changed during execution. This may be an asset. For example, messages can be given in a choice of languages by placing the translation in a file.
Literals are used when such modification is not desired. The name of the file mentioned above (not its content), or a physical constant such as 3.14159, might be coded as a literal. Literals can be accessed quickly, a potential advantage of their use.