Can't extract t($variables) for translation

There are several function calls like t($field['widget']['label']) which make the extractor.php script complain about "Invalid marker content", probably because these are runtime strings, and I can't figure out where the actual strings used in these variables are. How can they be translated in the first place?

Are these calls to t() useless? Or are there other t() calls which expose the actual strings for translation and these ones are not needed?