Real-world scenario: My uncle (who does not usually use spreadsheet applications) used Calc for a mostly textual table (I know that is not what it's made for, but lots of people do that). Not knowing that "=" triggers formula mode, he used "=" for "is", then typed words, left the cell and got "Err:509" as feedback.
He had no idea what that meant, and he is not used to googling for error codes.

I don't know whether a more explanatory feedback would have helped him in this specific scenario, but something like "The formula you entered is incomplete" would have been a lot more helpful than an error code. Error codes may be useful for support staff, but not as feedback to the user (and this is an error which is easily fixable by the user if they are clearly informed of the problem.